Changing to SNMPv3 in NPM 10.4.2 should NOT wipe out your historical data. I have changed from SNMP v2 to v3 in NPM 10.2.2, 10.3.x and 10.4.x with no issues.
I just double checked for you in a lab running 10.4.2 and the node has continued polling (see screencap). The double data point at around 18:55 is a forced poll after SNMPv3 was configured.
