Do it now and we’ll find out 
There is a high chance you’ll see errors like this:
reindexdb: error: reindexing of database "foreman" failed: ERROR: could not create unique index "index_fact_names_on_name_and_type"
DETAIL: Key (name, type)=(<something>, PuppetFactName) is duplicated.
or
reindexdb: error: reindexing of database "foreman" failed: ERROR: could not create unique index "index_fact_names_on_name_and_type"
DETAIL: Key (name, type)=(<something>, Katello::RhsmFactName) is duplicated.
Luckily the facts table can be wiped and will be re-populated by clients once the check in again.
For the first error:
# cat << EOF | foreman-rake console
conf.echo = false
fact_name_ids = FactName.unscoped.where(:type => 'PuppetFactName').ids
pp fact_name_ids.count
FactValue.unscoped.where(:fact_name_id => fact_name_ids).delete_all
FactName.unscoped.where(:id => fact_name_ids).delete_all
pp FactName.unscoped.where(:type => 'PuppetFactName').ids.count
EOF
For the second:
# cat << EOF | foreman-rake console
conf.echo = false
fact_name_ids = FactName.unscoped.where(:type => 'Katello::RhsmFactName').ids
pp fact_name_ids.count
FactValue.unscoped.where(:fact_name_id => fact_name_ids).delete_all
FactName.unscoped.where(:id => fact_name_ids).delete_all
pp FactName.unscoped.where(:type => 'Katello::RhsmFactName').ids.count
EOF
Then run the reindex command again.