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.

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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)