Like all creative works, costume design is a process. A lot of times what you set out to create is not the same as the end result, and it's often in the final fittings that most of these changes take place. Details are added. Costumes may need to be aged or distressed. Limited budgets mean working with what's already on hand and becoming familiar with the local thrift shops.
One of the coolest things about Rocky is that Sue Blane designed the costumes for the early stage productions and the film so you can see her work transpire from stage to film (and back to stage) - check out her 1979 interview on the Rocky Music website.
In the Roxy cast Kim Milford sported blue trunks. Looks like Peter Hinwood almost did in the film, too.
A test shot before filming shows Richard O'Brien's vest pinned together differently and without the blood stains - a look he would return to a few months later in the 1975 Broadway production.
Columbia's top hat got a new hat band, and in some shots you can see her bow tie also got a new band.
Almost nothing that Frank wears is pristine. There are holes in his fishnets and gloves, raw edges on his corset, repaired rips and blood stains on his lab apron, and ripped armpits and elbow in his dinner corset. It's in contrast to how clean-cut Brad and Janet are, so it adds meaning when Janet rips her slip.
Trying to emulate a "worn" look is tricky. You can cut the holes and leave the edges exposed, but it's still going to take time to get the complete effect; like letting some of the sequins fall off, or the fabric softening from wear. My point to all this is to have fun and enjoy the process!