EASA Part-66 licence does not carry any medical certificate requirement. Basically, if you have it, but not using it (like me), you only need to send it to your EASA licencing authority for extension every five years with EASA Form-19 - before it expires, of course. I do not see a reason to "surrender" one or even both of your licences to get a third country licence.
It is another story to get back privileges, as it all depends on Quality /Compliance Dept. of Part-145 organization, where you want to use your privileges again, as in EASAland you need both licence and Company Authorization to use your Part-66 licence privileges. First, you need to be current and prove that you were using your EASA Part-66 privileges for minimum 6 months in preceding 2 years (regulatory requirement) might not be that easy. (and other requirements, like having recent human factors/SMS training, EWIS and Fuel tank safety refreshers and of course passed succesfuly company assessment)
"Conversion" as such does not exist. It all depends on national regulations of individual state, what their rules are to issue you their licence. Mostly, they will give you credit for having EASA licence, but the minimum would be taking an exam on local legislation. But, it can be much worse too.