GCSE, number grades or letter grades? Or both?

I did my GCSEs in 2017 when the new number grading system was being introduced. That means some of my qualifications have grades in the old letter system (A*-G) and some of them are in the new current one (9-1). Now that I'm applying to university through UCAS, I'm not completely sure which way I should record them. Should I select the system that the grades were originally in, and have a mixed bag on my application? Or should I convert the letter ones into number grades?I get the feeling I should do it mixed, but I want to be 100% sure. I can't find info about this anywhere and I don't want to be rejected because of a dumb technical mistake like this.

You specify the grades as they were awarded. There isn't a one-to-one mapping between letter grades and numeric grades anyway.