I'd agree on using an HTTP redirect, but not necessarily a 302:

Note: RFC 1945 and RFC 2068 specify that the client is not allowed to change the method on the redirected request. However, most existing user agent implementations treat 302 as if it were a 303 response, performing a GET on the Location field-value regardless of the original request method. The status codes 303 and 307 have been added for servers that wish to make unambiguously clear which kind of reaction is expected of the client

You'd want to use a 303 if the browser supports HTTP/1.1, 302 of they support HTTP/1.0, and no status, just the Location: header if it's pre-HTTP/1.0.

You may have to change the logic of your program to deal with GET to the rewritten URL differently from a POST to it, to deal with someone being sent there the first time, vs. coming back to it later for an update.