Reviewing the original post I did notice something that I must have read over the first time - you included "frame" in the list of parts that need upgrading. That, along with the "fork" and the "wheels" (I'm going to assume you actually mean wheels and not something much simpler like tires), is pretty much, well, the whole bike.
And while I still like my 26'' and don't feel the need to buy a 27.5 because industry marketing or internet forums say I should, I also don't see the point in completely rebuilding a 26'' at this juncture either. For better or worse (and who knows, I haven't demoed a 27.5 - it may be "better") the standard is, as Banjo points out, now obsolete. When mine wears out, I'll probably just ride a 29er.
If you pretty much ride XC/Trail, and/or you ride a large or XL frame - then to answer your original question I would "upgrade my 29er to FS".
If you are a smaller rider, or if you like to do a lot of DH or all mountain type stuff, then you should probably go with your inclination to save up some dough and buy a FS 27.5.
However, if your 26'' frame doesn't really need to be scrapped, then I stand by my belief that you should give it some love and keep on riding it until you can get another bike you really enjoy riding.

Carry on.