If there were terrain changes, we'd have heard about it from Jeb. You finding diamond and lapis rarer? Placebo effect. Nothing more, nothing less.
Have you even tested these claims OP? Load up the seed in 1.7, go to the coords of the cave, see how big it is, and go to the same place in the same seed in 1.8 (Fresh world BTW) and compare cave sizes.
An even better method is to use Unmined, which makes it easy to see entire cave systems, to compare worlds generated in different versions, and in this case they are exactly the same (small changes to cave generation may leave some caves generating the same way; for example, increasing the size of cave systems just means adding more caves, so you'll still have the caves from before the increase, plus new ones; for example, if you look at the maps here, some caves are present in all of them except the no caves map, or in two of them).
Also, I looked at the decompiled code (using Java Decompiler) and both ore and cave generation were unchanged, unless you count dirt and gravel, which were reduced, and the new stone (all generated the same way as ores); the new types of stone generate in veins the same size as dirt and gravel with 10 veins of each type between layers 0 -79, compared to 10 veins of dirt and 8 veins of gravel between 0-255; here is an analysis of 400 chunks in MCEdit (note that dirt includes the surface layer, as I analyzed everything to the height limit, and is actually only about 25% more common than gravel underground):
(14:0),Gold Ore,3611 (9.03 ore per chunk, +10%)
(15:0),Iron Ore,38577 (96.4 ore per chunk, +25%)
(16:0),Coal Ore,74691 (186.7 ore per chunk, +31%)
(21:0),Lapis Lazuli Ore,1490 (3.73 ore per chunk, +9%)
(56:0),Diamond Ore,1376 (3.44 ore per chunk, +10%)
(73:0),Redstone Ore,10567 (26.4 ore per chunk, +6.4%)
(percentage is relative to what the Wiki states is the average)
Not only are diamonds not rarer, there are about 10% more diamonds (3.44 ore per chunk vs 3.09) than the Wiki's figures here, and a staggering 96 iron ore and 186 coal ore per chunk (the Wiki gives 77 and 142), no doubt because of all of the dirt and gravel they removed (actually, 1.7 shows an increase too because they doubled the y-range of dirt and gravel but left the number of veins the same for half as many in the same volume, plus a reduction in caves increased the amount of stone). Note also that the ore generator treats the new stone types as ordinary stone (the new stone is after all just stone with different data values), so ores generate in them like usual (they generate before ores but after dirt and gravel).
Also, while 400 chunks isn't a very lot, multiple analyses of different worlds confirm these findings; except for redstone in this case, the relative increases follow the pattern you'd expect based on ore generation order, from coal to lapis, as ores generated first prevent later ores from generating.