I've been doing work on 405 to 625 conversion recently. Darryl's Aurora SCRF405A and Frnak Cuffe's Hedghog [sic] both do a great job for 625 to 405 but just occasionally there's a need to do it the other way round. Either for working with historic broiadcast equipment or recovery of archive tapes.
I've now done this successfully with some existing hardware I had available. The whole story is here:
https://golbornevintageradio.co.uk/f...d.php?tid=7558
The "magic sauce" was driving the decoder chip with a different main clock frequency. Normally the Philips SAA7118 decoder uses a 24.576MHz xtal. I replcaed this with 24.576 * 405/625 derived in a FPGA.
The TVP5150 decoder as used in both Aurora and Hedghog converters has also shown some promise using the same trick.