What I always do is I have my yellow belt production that outputs into a chest which is then used for red belts. Then if you upgrade that chest to a yellow chest, the yellow belts from your trash slots are automatically put into it, you don't even need to set a filter.

But yes, if you are using bots you need a circuit condition so it doesn't use all of the items in the chest. To do that you only need a singular cable though, you directly connect the chest to the red belt inserter. Still the option I would recommend. Looks really clean.

I would just use a requester (yellow) chest and set it to either

1. Fetch a little bit, less than I'm using so it doesn't hog my stockpile during transition to all-reds

2. Fetch all of it and then treat it as singular place that I get yellowbelts from instead of where yellows are produced.

Both of these approached are yellow-to-red transition friendly but the latter assumes you're getting the belts by hand and not having them delivered to you by bots. You can jank rig the latter option to also serve yellow belts by putting a passive provider next to it and dumping yellows into that. Then once you're only using reds, just turn the inserter around so it feeds your yellow stockpile back into the system or just replace the chest with a provider so the bots move them.