What did they do to the OST?
They replaced most of the PS2 midi-based synth for the non streamed audio tracks with different samples.
The original FFX does have streamed audio with live recordings for 10+ tracks though. For some reason, To Zanarkand wasn't one of them. (Yes the Piano intro from FFX wasn't real piano but rather basically PSF-based midi synthesis instead of a live piano recording)
Based on the Besaid and Battle themes, the remasters seem to mostly be using VSTis and only a few live instruments, but today's high-end samples and soundsets are so much better than the almost 15 years old PS2 chip's soundsets.
Some people will dislike the changes, some of it due to nolstalgia, others simply the inherent dislike of change because change period.
For me though, being a regular user of FL Studio, Impulse Tracker and other music creation software, I like the fact they went to the effort of revising and using higher quality samples used for FFX, instead of just recording the original samples to mpeg / wav and just slapping them on the disc as is.
Although I suppose for some the option of having the original would be nice for those who want the choice to swap between the two.
Even if the original audio was to be included in mpeg form, I don't think disc space is an issue in this case.
I mean FFX was 3.4GB, so lets add an extra 10GB to account for revised textures and soundtrack, so that is 13.4GB right there
And FFX-2 was 2.8GB and without a remaster soundtrack and just revised textures, lets guesstimate at about 10GB (We need to still consider the original music is no longer 300kb midi but now in wave form so that will bump up the file size)
Even factoring 23.4GB (24GB going by info listed earlier in the thread.) for both games with higher res textures, and mpeg audio soundtracks, Bluray has a 50GB capacity. Space is a non-issue and they could have included the the original FFX audio easily.