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It is enough to specify which revision that is to be reviewed.

Is it possible to change it after the submission request? It would allow me to make some bugfixes (if necessary) while being in the queue.

I think bugfixes at that stage are fine: as long as there is a stable version available somewhere, and as long as all the reviewers are refering to the same version during the review - in other words try and keep things stable during the review itself. HTH, John.