You can migrate the LDAP to a new hostname by exporting the entire LDIF structure (as you mentioned) and then preforming a find/replace on the LDAP structure in a few different ways.
If your current host is "example.toto.fr" and you want it to become "example.titi.site.com", you would need to find/replace the following:
example.toto.fr --> example.titi.site.com
fr.toto.example --> com.site.titi.example
The tricky part is that in your NEW host, you have four "sections" (areas between the decimal points) instead of three, so you will need to actually create another dc= level manually to account for the additional levels and move those entries accordingly - a bit trickier than using another three-section host (say, if you were going to a new host called "example.titi.com")
You would also need to look for the call-outs for the nodes, such as "dc: example" and "dc=example" and replace them with their respective counterparts as well.
Note: there are several sections for windchill.ptc.com and com.ptc.windchill and similar dc: windchill and dc=windchill, etc. These need to remain unmodified.
My company specializes in all things Windchill. If you are looking for more assistance with this work, please feel free to reach out to me at firstname.lastname@example.org or check out our website at www.eccellent.com.