I forgot to mention it in my last post, but this is actually what I like about your idea. I don't really think I'd mind too much if players are able to throw around 10 fireballs at level 11, but your way allows lower level spells to have more punch while still requiring them to be lower level spells.I don't want actual higher-level slots because then you can cast more higher leveled spells. But free upcasting keeps 1st-level slots using 1st-level spells.
I also missed responding to your earlier post, I think limiting it to 5th level spells might be okay, but I'm also not sure how many 6th level+ spells have updating. Might be worth checking that out.
I'd probably also switch the warlock over to regular spell slots if your were moving this way. Remove mystic arcanum and just let them learn and cast higher level spells.