Plugin changes from 3.40 to 3.70

I've inherited a nopCommerce 3.40 site and am trying to get a plug-in working that supports v3.70 - 4.0.  I tried to add version 3.40 to the Description.txt file hoping it would show up as something that could be installed, but that didn't work.

Can anybody tell me what all changed between those versions and would there be any other tricks I could try to see if it "just worked?"

The pluign is from a 3rd party and is for a payment processor integration.  I'm waiting on their support to tell me if they have an older version OR if I can get the source code, but for now, I'm trying everything I can to see if it will work.

Any advice or tips would be greatly appreciated!

The dependent libraries (.dlls) have likely changed, and thus you need the source code to recompile it.  (Or, ask them to recompile it for you.  But it's possible it uses some services that may not be avail, or have been refactored.)