I do not understand why some think cutting down a ladder makes it unsafe (weaker?). Stresses will be *less* with it shorter and you will not degrade the rating if you do not do the modification stupidly. All you should need to do is set the ladder, measure the level cut points, remove lower feet and then reattach at the cut point. I have done this with some old degraded wood ladders to make much more stable short ladders and it was easy and came out well AND was much more stable. Some people are too fixated on bureaucracy with the certification comments. Take responsibility for your safety. You would know the rating (what it will hold) and use accordingly with any ladder. It does not matter if you are Type IIIa whatever if you do stupid stuff and do not set and use it correctly with reasonable competence.
That being said, long ladders are expensive generally. It might make better sense to sell the long ladder to someone who can use it and buy a shorter ladder that fits your need (with the right strength, weight, materials for the applications).