As Jerry said Natress do a very good value standards conversion filter that I’ve actually used myself, one word of warning though, due to everything the filter has to do it is VERY time consuming, from what I can remember you’re looking at a ratio of 1 to about 11, so a 60 minute programme is going to take around 11 hours to convert, the results though, for the price asked, can’t be faulted.
It’d be so much easier with two mouse buttons!