It seems that migrating/moving from WHMCS to Blesta and from Blesta to WHMCS seems not to be that big issue.

Thanks to Blesta Import Manager plugin and thanks to WHMCS Import Assist.


WHMCS import assist:

WHMCS ImportAssist supports importing data from the following applications and versions:

WHMCS Versions 6.2.x, 6.3.x, 7.0.x, 7.1.x, 7.2.x, 7.3.x, 7.4.x
Blesta Versions 2.x, 3.x, 4.x
ClientExec Versions 5.3, 5.4, 5.5
HostBill 2016 & 2017 Versions

Blesta Import Manager:

Blesta 2.5
WHMCS 5.2 - 7.2 (Reported to work with 5.x, 6.x, and 7.x)
Clientexec 5.5
CSV (Obtain from the forums, imports clients only)