There was 6 gears on the rear, I removed that cluster and put a Sunrace 7 on with a 34 tooth bail out gear, I did this thinking future when I would be touring across the US loaded and may need a really low gear to climb a steep grade. I have the original gears, they only had about 800 miles on it, the new gears had about 100 miles on it. The factory original chain is being used with the new gears and worked fine. The Mountech derailleur also had no issue going up into the 34 gear from 24, in fact it was as seamless as the original going from 23 to 28! that actually surprised me, I thought the derailleur would have to work at it make the jump but nope, no problem.
Note; the specs from the 1985 Schwinn catalog said it came with a 5 speed cluster, mine is was all original when i bought it, I knew the guy, he bought in 86 on sale, he rode it about 200 miles when he fell at his house off a roof and severely injured his back, he kept it covered under blankets in his garage all those years until 7 or 8 years ago when a conversation he and I was having revealed we had the same goal, to travel the US, he could no longer do that so he sold me the bike so i could do it. It had the original seat and tires even, it was in showroom condition when I got it. Another odd thing about the bike was the catalog said it had 36 spokes wheel on the front and 40 on the rear, mine had 40 front and rear; I think that 40 spoke wheel screwed me when I hit the barrier because the darn wheel didn't even come out of true even the slightest, had it been a 36 or a 32 it would have maybe collapsed saving the fork? I did remove the original pedals and put on a set of Shimano 530's, but the seat I left on because the Avocet was comfortable to ride on.
The other change I made was after trying to adjust the brakes myself but with zero experience with cantis and getting nowhere I took it into a shop, they put on a set of new Jagwire cables (even matched the original goldish colored housing) because the old brake cables didn't have the slicker internal technology of modern wires which made the cantis work stiff; cleaned and lubed the all the pivots; and put a new saddle wire of different length, can't recall if they went longer or shorter, just one size different; and they replaced the brake blocks with Kool Stop Salmons; the brakes worked a day and night difference afterwards.
That's the only changes I did to the bike, I even used the original rack for the first tour but later switched to Tubus stainless because I didn't trust the original Blackburn aluminium one to hold up forever.
The only thing I didn't like about the bike, and this fault all touring bikes had back in the
day, was that it didn't have water bottle mounts on the fork, so my touring bike will have those; also it did not have a third water bottle mount on the bottom of the down tube, but again this was typical of all touring bikes back then, so I added a third one via a clamp system specifically made to do that.
Of course I didn't use the original tires because flat protection is so much better today so i went with Schwalbe Marathon Greengard tires, but I have the original Passage tires, they're not cracked so they can still be ridden.