btree_gist extension - gbt_cash_union return type

I've noticed a discrepancy in the return type for the gbt_cash_union
function...

On fresh instances of postgres 9.6.11, where the btree_gist extension is
newly created (version 1.2) yields a gbt_cash_union function with a return
type of gbtreekey16

While instances that have been upgraded from 9.6.2 to 9.6.11, where the
btree_gist was originally installed as 1.0 and then upgraded from 1.0 to
1.2 - that same function has a return type of gbtreekey8

I expect something is missing from the extension upgrade script to modify
the return type.

Is it safe/recommended to modify this function to return gbtreekey16?
Perhaps safer still to drop the extension and recreate it?

Thanks in advance. If this should be considered a bug, and there is a
better place to report such - please advise.

Tim