The member number isn't reset in 6.7.3, it's reset in 7.0 as data is migrated from flat files to a database.
Yes, when upgrading to 7.x all gaps will be removed, all member numbers will be changed.
No matter what, the member numbers will be changed; as member #1 in threads is the "guest"/"anonymous" user. That is unless you had no member #1 and hacked the importer to not re-assign id's... I've not played with the importers enough to specify how to do so however.
does the system keep handing them out from the highest available number allowing the gaps to remain?
That one