i don't no much about blitz+, cause i don't own it and i don't know if it's similar to blitz 2d
FYI, as far as the superficial stuff goes (i.e. command set), BlitzBasic (Blitz2D) is nearly a subset of BlitzPlus. That is to say, pretty much any program written in BlitzBasic will work in BlitzPlus with very few modifications necessary, if any.
However, under the hood, they're very different. So, in some cases, it may be advantageous to re-write Blitz2D programs with the BlitzPlus paradigm to make the program more system-friendly, optimized, and such, just because of the way things run underneath.