1. Dump your user entries from the old machine in LDIF format. Update all parts of DNs and attribute's values which are bound to some unique id of this specific instance with some stream text editor, like sed or awk. Then import them into the new server. **Or...**
2. At the old instance, make OpenLDAP listen on external interfaces. At the new instance configure Cache Refresh and point it to the old instance. Set mappings for all attributes you want to enter, and perhaps write a CR script to additionally update some attributes on the fly, if needed.
The most difficult part will be migration of users' passwords, if you stored them on user entries in Gluu's internal LDAD db. As 3.0.1 used OpenLDAP which uses bcrypt for hashes by defaul, and 3.1.2+ use OpenDJ which doesn't support it. So hashes of passwords become of little use, even if you'll manage to import them into 3.1.3 instance. There is no solid solution for this issue, only [a workaround of sort](https://github.com/GluuFederation/oxAuth/issues/753). It's a custom script which allows such imported users to login by checking bcrypt hashes inside the script itself. So, at best, you may need to do some scripting, adapting this solution to your own needs (we can't support you here as a community user)