Is it possible to get a downgrade license to 2015 PRO 64 bit from my curent 2018 Pro?

A GT 730 does have a max. power consumption of 49 W, a GTX 1050(Ti) of 75 W. What makes it very likely that your exisiting PSU is capable to provide enough wattage for the GTX 1050(Ti). Even if not would a new, reliable PSU with ~500W from e.g. Corsair available from roughly 60.- bucks.

Downgrading is not available by Trimble… being required to support an old version with issues already fixed by the recent version is not worthwhile the effort.

For using SU with the latest operating system versions and for profiting from hundreds of problem fixes using the latest SU revision - besides new functionality - does obviously make sense, especially if the investment for doing that (see above) is that small.

more info here.

btw, if a downgrade would be feasible version 2016 would be the latest version with an option to render OpenGL in software (see above).

without having a choice upgrading the hardware appears to be without alternative… but you can always contact Trimbles tech. support and ask for a downgrade to v2016 because of your limited hardware.