As of that update, my older world, started as of around 3 years ago, will no longer load. Instead, there is a constant stream of crashing.
As for testing, I'd open the world in the snapshots following, knowing it would crash, and would repeat that step a few times over. Then go load it in a snapshot, or update that I know would support it, and every time I noticed more and more of the chunks would be converted, or reset to natural, slowly deleting everything I had worked so hard on. The only sort of progress made in the hours of testing is, that if I take that world backup and put myself in the end before loading in a snapshot that corrupts the world, when loading the world corrupting snapshot, i'll actually be able to fully move around in the end, and when jumping in the portal back to main world, it'll load fine, and after so many blocks, maybe 500 it'll only then crash and corrupt it forcing me to load backup and try again. I'll paste right below the crash report I receive when trying to load back in after a corruption.
---- Minecraft Crash Report ----
// Who set us up the TNT?
Time: 2/16/18 3:24 PM
Description: Exception ticking world
i: Non [a-z0-9/._-] character in path of location: minecraft:Skeleton
A detailed walkthrough of the error, its code path and all known details is as follows:
-- Head --
Thread: Server thread
-- Affected level --
Level name: Chew's Minecraft World
All players: 1 total; [rz['ThisIsChew'/260, l='Chew's Minecraft World', x=401.67, y=69.42, z=-230.92]]
Chunk stats: ServerChunkCache: 4715 Drop: 0
Level seed: -1505485270
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (-172,64,252), Chunk: (at 4,4,12 in -11,15; contains blocks -176,0,240 to -161,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 62896695 game time, 27028097 day time
Level dimension: 1
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 97108 (now: false), thunder time: 46003 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
-- System Details --
Minecraft Version: 18w07c
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 400750784 bytes (382 MB) / 1056178176 bytes (1007 MB) up to 17166499840 bytes (16371 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx16G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [rz['ThisIsChew'/260, l='Chew's Minecraft World', x=401.67, y=69.42, z=-230.92]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.
I know for you particularly this will sound like a broken record but, don't expect snapshots to work perfectly. Mojang says to keep backups of old worlds when trying them in snapshots, and luckily you have. This is because when snapshots are developed, they are for testing out new features and changes. Compatibility measures and fixes generally come late in the snapshot process.
I saw there that some guy had the same issues, and it was closed and redirected to a following issue that was marked as solved. But I couldn't understand all to well, how they solved the problem. So maybe you can read the redirected article and provide much appreciated support.
Solved or resolved simply means they will apply possible fixes in a future update when it comes to snapshots. Again they are very clear about possible corruption to worlds in the snapshot phase, its part of the process. You should only worry if its still an issue when when the completed version releases, or when it goes from snapshot to a 1.13-Pre-release build. As they decided to merge the 1.13 version and what was originally the 1.14 version, its still fairly early in the development process.
Best bet is to test a clean backup/copy of the original in each new snapshot version to see if it persists, if it does open a new bug report for that specific release. Also if you have not already, make sure to run all snapshots in a custom game directory (not .minecraft) as some changes to the games assets and libraries will cause a few issues jumping between versions.
It's nice to know that solved means there's no solution, rather its acknowledged. But I don't agree on just waiting it out until official or pre release because if at that point it's still persistent, then it would of been a waste of how ever much time I put in from here to then. I read someones comment, forget where I read it, that with the new world generation they're putting in it has a potential to ruin some older worlds. And after just getting this world back after a few years lost on a hard drive in storage, I would rather get an idea of start a new, or continue old sooner then later. But I guess I'll just hope someone can help find some workaround other then some janky mcedit copy and paste project,
They are not marked as solved, they are marked as resolved, and if its a coding issue, there is nothing they can do to change that in the current version, it requires and update, marking it as resolved means they will address it down the line. Also while similar the report you linked does not match the error you are getting, if you cannot find a similar one, in the latest snapshot, make a new bug report for it.
New world gen does not get applied to areas already made. The world you have is already created and saved, once you progress beyond that area, you can expect issues along the border of old and new like this. Which is basically unavoidable.
If the crash marked that chunk as corrupt, which it seems to be doing, it will be reset as you have seen. Once the issue is resolved, it shouldn't happen anymore. I have seen a few other people with an error much closer to yours, but cant seem to find a bug report anyone has posted with it, so I would suggest reporting it with your logs and info, so the devs can look into it.
Until then, as the issue seems to stem from a skeleton entity, load a copy of the world into a version you know it works on, ideally 1.12.2, then try killing all the skeleton entities in the area you know gets corrupted with a command (I have not used vanilla commands in a while so you will need to figure that one out). After doing so, see if the error or world corruption still happens in the latest snapshot.