I am not a lawyer
IMHO, going from GPLv3 to GPLv3+ is a license change, perhaps legally as important as going from GPLv2 to GPLv3. However, GPLv3 and GPLv3+ are compatible licenses.
If your contributions are small, that is some patch of a few dozen lines, they should go into some existing source file, and I guess that you legally cannot change the license of that existing file without asking formal permission by all copyright owners and contributors.
However, if your contributions are significant, and you can arrange most of them to go into new source files, you could license your new source files under GPLv3+ and keep the other (patched) source files with the same old license (e.g. GPLv3).
But I could be very wrong (because in some way, I'm making the implicit assumption that a license apply to a source file, not to a free software; perhaps that is a mistake).
The practical advice by Zim i48 of contacting the other authors is probably the best.
BTW, don't do anything without asking (or at least seriously trying to contact other authors). Such a license change might be considered as important by some authors, and if you change it without permission these authors would at least be pissed off (and if paranoid could perhaps even sue you).
Note *. Some weird implementations of Smalltalk are only modifying some "system image", then the notion of source file has no sense (and the entire source code is embedded inside that whole system image).