First step is upgrade to 1.4 on your old system. This should work without problems if all changes and extensions done are inside the local structure.
Check if all checks are working as expected and fix problem if existing.
The step from 1.4 to 1.5 needs significant more CPU power from your monitoring system, keep this in mind for the next step. Other question is as this is a fairly old system with 1.2.8, what operating system do you use there?
Only with upgrade i lost no performance data until now and i upgraded in the meantime over 200 systems. But backup is every time a good advice 
Why “synced” as it is a standalone system at the moment, it will stay a standalone system also after the upgrades. To integrate such a big system in a existing distributed monitoring is not easy.