Shorten Sleeves on Store-bought Sweater

I know how to shorten knitted sleeves, but I’ve only attempted it once and gave up. The store-bought sweaters just aren’t made like handmade ones. I have some new ones and the sleeves are 6-8” too long! I usually turn them under and keep them up with elastic bands on the inside (I have some gold bracelets made for this and I made some out of 1/4” or 1/2” elastic bands.) I’d really like to fix them permanently. I’m afraid to cut and have short lengths of yarn to work with or ruin the sweaters completely. Have you successfully shortened knitted sleeves and have tips for me? Thank you!

It helps to take a close look at the seam and see if you can tell if the sweater was serged together or seamed more like a handmade sweater. If the sweater has been cut out and seamed, you’ll certainly end up with short lengths of yarn.
These videos are for thrift store sweaters but the cautions apply to any sweater.
http://dawnprickett.blogspot.com/2008/01/recycling-sweaters-for-yarn.html

Thank you!