How much validation work you need to do does not depend on the transition from an HPLC to an UPLC instrument, but on how many parameters you are changing, and what the level of change is. For general methods, I would look at the guidelines of ICH and FDA.
To be clear, if you have a method that runs on a 2 mm column at 1 mL/min on a standard instrument, and you want to run it on the same 2 mm column at 1 mL/min on a UPLC instrument, the only thing to do is the qualification of the instrument, in the same way as you would when changing HPLC instruments. However, I do not think that this was your question.
I personally would be much more concerned changing a method from packing material A to packing material B than going from packing material C on a standard HPLC to packing material C with a smaller particle size on UPLC, or vice versa. Therefore I do not see a fundamental issue tranferring methods to and from a UPLC instrument.
A change from one packing material to another (from HPLC packing A to UPLC packing C) should be treated in the same way as you treat a transfer of an HPLC method from one packing to another. I believe that in such a case, a full validation is needed.
If you are looking at a USP method (where the packing is fuzzy), there are clear steps prescribed for changes in column length, column diameter, flow rate etc. Generally, the steps prescribed by the USP are much more restrictive. You will not be able to change from a 30 cm x 3.9 mm microBondapak C18 column to a 5 cm x 2 mm UPLC column without revalidation.
