You can always play older maps on the newer versions. It's the opposite that would cause you problems.
Remember that you don't need to buy Reforged in order to use the latest patch and play on the BNET.
All purchasing Reforged does is give you access to the HD graphics. Do note that there WILL be bugs on the latest patch. But if you remain on SD mode you can avoid a good portion of them.
I made a big corrections for this fact. You did assume all maps and campaigns are fully playable on Reforged? Apparently not.
Some custom campaigns which were updated recently are only playable for the latest patch, but NOT for the Reforged. However, I heard that if you play the custom campaigns on Reforged with the most recent one (1.36, I think), you might still encounter several bugs that was never fixed at any time for the specific campaigns. For instances:
1.
Legends of Arkain is prone for game crashing when you have the latest patch, unless you used the recommended version (which is 1.29.2).
2.
Heart of Storms: Dusk of Draenor doesn't seem to work on Reforged because it uses the custom game files made in MPQ (and Reforged uses CASC). The recommended version for this campaign should be 1.27b.
3.
Warcraft II Remakes (LordPerenoldeII remake ones, not the ones by Helldoom). You are stuck at the point where you can't harvest lumber (the amount of lumber is not increased) along with the other problems. To play this campaign without problems, you are going to need 1.28.5.
So far, those are the only campaigns that you should not play on Reforged because of the bugs you might encounter. This also includes the campaigns that no longer received a further updates such as Arthas Campaign, The Chosen One, and Wanderers of Sorceria.
I would also point out that CSW Beta (Chronicles of Second War, a Reforged remake of the Warcraft II) is NOT working on latest Reforged patch due to the major bugs.
It's hard to say whether you should downgrade your Warcraft 3 for some custom campaigns or not. It depends on how you value the maps you are playing.
