SA-MP 0.3.DL is a seperate version of SA-MP featuring server-side custom models. Right now, custom objects and custom player skins are supported, with more types planned for the future.
Question, will the downgrade still be necessary to play this version of SAMP? Probably a stupid question, but a question nonetheless.
San Andreas Multiplayer does not support backward compatibility, unfortunately. Therefore, you will need to install the 0.3.7 client to play on 0.3.7 servers; 0.3.DL client to play on 0.3.DL servers; and the now-discontinued 0.3.8 client to play on 0.3.8 servers.
In the case of Bay Area Roleplay, which is based by SA-MP 0.3.DL, you will need to have the 0.3.DL client to connect to the server.
i wouldn't call it a downgrade and swapping versions is as simple as switching your samp.dll. Just have one for 0.3.DL and 0.3.7 if you play other servers.