Use ‘reader’ URL structure in ‘transitional’ mode

I’d like to use ‘reader’ URL structure in ‘transitional’ mode, i.e.: for my permalinks to be appended with /amp instead of ?amp. The main reason is because we’ve been running in reader mode for a short while and the URLs have been picked up by Google so we don’t now want to change it.

I think I can do it myself using add_rewrite_rule() but don’t want to do so if there’s some method already built in that I might be messing with. I also don’t know if there’s an important reason for using one over the other.

Many thanks