Fortinet 100D firmware 5.0, build128 (GA)

Or look into the upgrade matrix for older versions:

https://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD45543&sliceId=1&docTypeID=DT_KCARTICLE_1_1&dialogID=151956038&stateId=0%200%20151954796%27

Link is visible in the FortiOS Version Upgrade Path tool page...

hint: download the corresponding Excel sheet and filter for your version.

Given that v5.0.0 (b128) is 8 years old, I'd rather install a newer version bluntly, and rebuild a valid config, than upgrade each and every minor version up to v5.4 or v5.6. There have been major changes (i.e. between v5.2 and v5.4 IIRC) which need manual configuration anyway.