No. Once a Flexible volume is converted to FlexGroup, all existing snapshots will be marked pre-conversion and cannot be used to create clones.
Following error will be displayed when attempted:
cluster2::*> volume clone create -flexclone clonevol -type RW -parent-vserver svm1 -parent-volume siva -foreground true -parent-snapshot snap1
Error: command failed: Cannot create clone from Snapshot copy "snap1" because the Snapshot copy was created when volume "siva" in Vserver "svm1" was a FlexVol.