Hi,
I encountered this. I'm not familiar with technical reasons but this is one of the pre-upgrade-validations.
I suggest using advanced upgrade on MLM. During export you can use -l flag to export logs as well as DB. No point of exporting indexes as well since R81+ uses newer version of SOLR that uses newer set of indexes anyway. This will reduce export time.
After exporting you can suspend/stop MLM VM and import to the new VM with one larger vHDD.
This way you do not need any additional actions but the upgrade itself and the creation of a new VM (+OS installation and first time wizard building another MLM).
Hope this helps.
Kind regards, Amir Senn