I submitted a proposal this morning that was missing some relavant information which wasn't available when I initially submitted. Unfortunately, Upwork does not have the ability to append to an existing proposal; the only method is to withdraw and resubmit. The client had not yet read my proposal so I went ahead and withdrew it, and informed the client that I would be submitting a new one.
I prepared my new proposal with the additional information and attempted to submit it but I got the message,
"Since you withdrew a boosted proposal for this job, you are no longer able to apply."
I don't mind Upwork rules but I wish I had been provided some kind of warning or popup that let me know that I would not be allowed to resubmit in which case I would have let my original proposal stand. I literally have performed the exact same job that the client has requested and I think my contribution---whether accepted or not---would have been valuable for this client and it is a loss all around.