It would make sense to allow converting a table from implicit to explicit row start/end columns. Like this:

create table t1 (...) with system versioning;

alter table t1 add column rs timestamp(6) as row start, add column re timestamp(6) as row end, add period for system_time (rs,re);

This would allow to add indexes on these columns post factum, when the table history grows, and still preserve the history.