the best is to talk to the BS. but from what i have gathered, BS usually stocks outdoor OTR gowns for PS, and indoor OTR gown for AD. If you are doing solemnisation and AD on different day, there are ROM gowns to choose.
OTR gowns design won't change but there are likely new designs to choose as time passes by, bcos OTR gowns are usually gowns brides MTM and after they worn it for their AD, the BS takes it and uses it as OTR gown.
Why don't you start visitng BS, then you will have a clearer picture since different BS has different packages?