I'll be different here ( I often am!). If the metal is in great condition there's not necessarily a need to replace it until it shows deterioration. This means carefully lifting shingles and looking where you cannot see. You stand a higher risk of introducing leaks when you disturb something that is not leaking given that it's all in good shape so I'm for leaving it alone- it ain't broke (that part anyway) so don't go fixing it! Instead look at the same kind of units that you have now and see how the rubber goes on. If it can be replaced with the pipe inserted then see if you can buy just the rubber parts. If not buy the whole darn things and swap the rubber parts out, remembering to never use their products again.
This is the exact reason I use only Oatey pipe flashings- they sell replacement rubber and it can be easily installed without disturbing anything else. It's been my experience that no rubber flashing parts last over about 10-12 years in the hot summer sun here in SC but the metal or plastic parts last at least twice that long or longer. The downside of this is that nobody stocks the replacement rubber; you'll have to order it and you may discover that you have to buy in lots of ten in each size, but it's not a problem for me since I'll use them somewhere down the road.
The 'collars' do work well as long as they cover the entire top of the existing rubber. I install these over a bed of NP-1 roof sealant in a caulking tube which helps seal the cracked one underneath. These last about 8-10 years here on smaller pipes but about half that if stretched over larger sizes in the range of fitment given. Not a "proper fix" but can be just fine, especially if the roof is nearing it's end but not quite there yet so not worth spending a whole lot of money on. Replacing the original rubber is much better where it's possible
BTW, "carded" means that there are sold on display cards that one can hang on pegs like you see stores doing- these are usually one per card as opposed to loose stock not meant for display. Same stuff, different packaging.