What you are looking to upgrade from Magento 1.5 to which version?
If you are looking to upgrade 1.5 to latest magento version 2.4 then there will be a lot issue you will get it as Magento 1.5 very old version. Or you can try once let us know if you will face any issue.
I will suggest to go-ahead with incremental upgrade.
First move vr 1.5 to vr 1.9.x suitable
Further if everything goes well its easy
In case you need help let me know
I had also some old magento installs, at the end I decided to do a fresh install of magento-lts which is version 1.9 with security updates. Do the same, do not copy some crap that could be in there. (Eg my fresh install of magento-lts did not work 100% with the original 1.9.?.? database, probably some **bleep**ty module corrupted it)