my advice would be finish working on active projects at 16bit, but set up new projects at 24bit exclusively.
the main point in the process where you gain from 24bit is the recording process, anyway. the mixing and processing in the DAW depends on the DAWs precision, and current DAWs have an internal precision of 32bit float or 64bit float. you could export mixdowns in 24bit (even when 16bit recordings have been used). but i have the impression that mastering engineers like to get the mixes in whatever format they've been recorded natively.
converting existing recordings in a project from 16bit to 24bit won't change that much, i fear.
but for new projects, with new recordings, you can truly benefit from the extended resolution of 24bit.
sidenote: if you record carefully (with appropriate levels etc.) the difference between 16bit and 24bit won't be huge unless the signal is very dynamic.