If it is a complex project, I usually would avoid a rewrite. There is good literature on how to approach modernization of legacy applications step-by-step. Read

If it is a complex project, I usually would avoid a rewrite. There is good literature on how to approach modernization of legacy applications step-by-step. Read it, apply it. In the end the modernization is a re-write. It's just one where you don't loose features. For webapps you can even do this while changing languages (with a router that delegates some things to the old api and others to the new api).