Bitcoin is designed so that different asymmetric crypto governing controlling coins can be compatibility introduced-- as has been done a couple times in the past (to introduce revisions to the script system). PQ crypto could be introduced in that way.
Unfortunately existing PQ signature schemes don't have a great mix of maturity and performance that make them especially attractive absent a clear and present need.
For years I maintained a private patch set to introduce hash based signatures on short notice if needed. I haven't for a number of years now in part because there are now enough contributors I'm confident that if it were needed it could be done quickly without any special preparatory effort.
Users can change the rules governing their coins just by moving them.