I see nothing wrong with wearing the Badge of your disbanded Regiment. We wear our Glengarries with Badge at the parade for our special Battle Honour (Minden) and reunions. I still have and wear my Glengarry with either the original KOSB cap badge (now 1 Scots Borderers) and sometimes with the Badge of the Lowland Brigade to which the KOSB belonged for the few years of the Brigades existance.
Go ahead and be proud to show them all that you served in a Proud Regiment.