The only practical reason for re-flagging (I guess you really mean re-registering) a boat is if you as the new owner is not eligible to keep the registration
(say if there is a residence or citizenship issue, as I believe there is with an American keeping UK registration
in the Registry of British Ships). Legally, you do have to transfer the ownership
with the registration authority and if youíre not eligible they will require the previous owner to de-register (you canít de-register someone else, otherwise crooks would do that all the time).
An emotional reason to re-register is wanting the boat to represent a country you care about.
If that doesnít matter to you, which is cheaper, US or Malaysian? Neither matters if you return with the boat to the US, as youíll need to pay local taxes then and they donít depend on the flag of the boat.
You can check for liens against the boat by checking itís current registration - any mortgages on the boat will be shown. Those are the only ones you need to worry about.
All that said, the UK registration is an easy and inexpensive one to maintain and your UK ships radio
licence is free. But you do need to qualify, so check the rules. Is the boat registered in the UK proper, or one of the offshore ports
like Jersey or Isle of Man? If the latter, there may be a few more options to qualify for UK registration.