You will be doing just fine with the stashed Dual Duty. Before using old threads, do a yank test to see how easily the thread breaks. If you can easily break it, don't use it! Otherwise, good to go.
Some of those old vintage machines are lovely. I'm having various issues including I dropped the foot of my vintage Remington and something broke, which is why I am down to my portable "class" machine. Finally got it open and was impressed by how complex it was inside -- but it is definitely major part broken and not repairable by me and I didn't like seeing that little flash of electric spark that must have happened when I plugged it in to see if it still worked... I'll be able to get a new foot pedal for it. I have a vintage Pfaff that was my favorite machine from when I was still doing costuming, unfortunately, it is currently more of a TV stand than a usable sewing machine but that can be semi-easily changed.
So most of my old wood spools of cotton thread are no longer good -- but some still are! Haven't come up to a Dual Duty that wasn't usable yet, and a lot of what I use does come from estate sales and such. I used to collect vintage fabric with the intent to sell it but then realized it was only coming into the house and not leaving... also bought a lot of thread during that period.
I've been a quilter for 40 years but I have progressive vision issues and I am relearning how to knit. At least I think I'm knitting, it might be a purl! All I'm doing right now is I have a bunch of yarn that I am just doing the same stitch and it will eventually end up as a dog blanket. The beginning part is super rough, but I'm quite impressed with it having been at least 20 years since I last knitted and without my contacts I'm legally blind.