I think I've found out the correct status so far.
It looks like the eastbound lanes of the new section have opened up, as well as the direct I-45 connection to those eastbound lanes.
If you don't exit at the new US 175 exit and try to use S.M. Wright instead, there won't be any US 175 signage at the exit. You would be able to travel as far south as the Elsie Faye Heggins (ex-Hatcher St.) exit, then you would be diverted off on a detour. I haven't heard whether the detour is down the service road, or off onto side streets.
Coming westbound on US 175, there is an exit you must take (I'm not sure if it's Bexar St., or a new one for S.M. Wright)... you aren't yet able to drive on over with the new lanes to I-45.
I don't know if there's any new permanent guide signage yet on any of the open new sections, or if it's just temporary construction-style stuff.
For those coming north on I-45, there are no direct exit ramps to eastbound US 175 (it would be nice if that does get rectified in the future). There will be (if they're not done already) new or redone ramps to Lamar for northbound I-45 traffic, in the vicinity of the new US 175 interchange.
Someone has already updated Google Maps with what is open:
You do not have the required permissions to view the files attached to this post.