I don't have an answer to the dilemma of swapping your extra thread, but I do have a suggestion for getting a variety of colors without buying a bunch of spools. I bought a bobbin donut full of bobbins pre-wound with a rainbow of colors and I use it when I just need a little of a matching color.

If you decide to do that, be sure to check that the bobbin donut has the type pf bobbins your machine uses. They are not all the same.

Here is an example (this is not the brand I have) https://us.amazon.com/SIMTHREADS-PREWOUND-Polyester-Embroidery-Bobbinsaver/dp/B07K2YSNNK/