Most of it certainly works as canon for OTU. Authors were required to regard previously published information as canon for the GTU, so most of what was produced is compatible with OTU canon. (I say 'most' because there were some mistakes made). Apart from that, anything that's established about the GTU prior to the Change Point should, conceptually, apply equally to the OTU.
I put it a bit earlier than that, although I can't be precise. The earliest difference I've spotted is that in the GTU Strephon decides to elevate Norris to archduke early enough for Norris to leave for Capital in time to get there by 001-1117. In the OTU he postpones that decision (or decides against elevating Norris).
Hans