Is it a difficult task to move customers from one reseller host to another? I think I need to find a different host.

It is not too bad of a task if you can get the old host to paticipate. H-Sphere has a XML plan/user backup/restore feature that can be used, however the old host has to be willing to dump those to an xml file for you.

Of course this feature is only available in version 2.4+. If you can get that done then the new host can import those xml files and you just have to copy the databases and domain files across to the new accounts and setup any admin settings you had in the CP.