Would there be any adverse impact in changing resource class DB2 from ATTR(NOMASK) to ATTR(MASK)?
No, there is would be no impact from changing the class to ATTR(MASK).
You just need to make sure to:
1. TSS REVOKE all permissions
2. TSS REMOVE the ownerships
3. Change the class refinition to ATTR(MASK)
4. TSS ADD the ownership back again
5. TSS PERMIT the resources again