If you’re using anything that uses Java’s built-in hotswapping this will not be possible. Unless you use tools that extend that (somewhat hackily) like JRebel, which is commercial, or a completely different JVM - eg. DCEVM.
Or you could use a tool that’s been designed to do live coding with Processing from the ground up, like PraxisLIVE (I’m biased obviously! 
). Or if you don’t mind using Ruby instead, there’s also Visor.