well, if **I** were to shorten shorts, being the cheapskate that I am I would reuse the elastic. If I was taking enough off that I could just cut the elastic off, then I'd put on new. Or, if the elastic is toast from picking it off there, I'd replace it.
any number of the mail order sources listed on the site sell gripper elasitc, it is about $1 a yard.
as for comfort, I haven't purchased commercial shorts in 20 years, so I have nothing to compare to. A friend of mine just made some of these and says they are super comfortable for her. Everyone is different.
2015 Liv Intrigue 2
Pro Mongoose Titanium Singlespeed
2012 Trek Madone 4.6 Compact SRAM