Turning my 2014 IG rMBP into a 2015 DG with a new MLB?

I have a 2014 rMBP that has the Iris Pro 1536MB IGP setup, and it feels like a tired configuration. The problem at the time I got mine was the 2014 DG was not easy to come by, and the 2015 DG was a lot more expensive then the 2014 IG :/.

I’m looking at possibly turning my 2014 into a 2015 with a used DG MLB, but keeping the screen and upper case which are still fine. Is there anything preventing me from doing this, or am I better off seeking a nice 2015 DG without the SSD for almost nothing, moving my SSD over and reinstalling MacOS? Other than the massive AG issue, the rest of it is in very good shape.

@danj pretty sure it can't be done right? I know I can do a 2014 IG>DG upgrade but once Big Sur is no longer viable I have to move up again anyway.

It's probably better to upgrade model years, not within the series.