Installing the same crate multiple times with different versions?

You would probably have to have two separate proxy crates to handle old/new logins and a way to either detect which to use, or try the other on failure of the first.

So your dep tree would look something like:

[your crate]
     |
     |-> [argonrs_old]
     |       |
     |       |-> [argonrs v1]
     |          
     |-> [argonrs_new]
            |
            |-> [argonrs v2]