I agree with Heisenbeg above. They just look tarnished to me. For removing the tarnish, you can use a polishing cloth or a chemical silver polish and a regular rag, but this can take a lot of time rubbing to polish. There's another way I tried once on some solid silver pieces. I don't remember the particulars, but you made a solution of baking soda and water (and maybe some other stuff - all regular kitchen ingredients), and boiled it with a piece of aluminum foil. Probably best to do your research before you try it and make sure if it works on plate. Your best bet is to take them to a jeweler to get them professionally polished. This will cost a little, but not that much, as it's only about 10 minutes of work for someone with professional equipment. If you do, be sure to tell them it's plated so that they will go gently with the buffer.
Just a quick note if you decide to go to a professional jeweler ... find a shop that does their own repairs. A small family owned shop or one that caters to the art/design type of customer is usually best. If you go to the mall shops, they will often send the piece to a small shop, and then charge you a middleman markup for the same work. (I used to work for one of the little family-owned jewelry shops, so I've seen this from the other side.)
As to the question of sizing .... sorry, but plated rings can't really be re-sized. Resizing a ring usually involves cutting a piece out and soldering the ends back together; or alternatively cutting it open and soldering in a new piece if you're upsizing. Either way, there's a good deal of filing involved to smooth out the solder joint and make it invisible. This filing will quickly cut through the plating and leave a rough edge where the base metal is exposed. Yes, it's technically possible to resize and then replate, but it will be difficult to find a shop that has the equipment for this. Considering it's silver, and not gold, my guess is that you would end up paying more than the ring is worth to have it sized and replated - even if you can find someone who can do the work.
If you want to keep the Buffy/Angel rings nice, but also want to wear a claddagh, I'd recommend getting the Buffy ones polished, and buying yourself a set of solid silver ones to wear. Plated jewelry doesn't stand up to continuous use very well. The plate often begins to wear down in the parts where it gets bumped all the time, and again will leave a rough edge where the base metal is exposed.
If you want real silver rings ... again - don't go the mall store. You will pay way too much. There's a silver jewelry shop in my local mall (Seriously Sterling, or something like that) which basically sells silver jewelry for the price of gold. It's pretty unbelievable what they can get away with charging.
While you're getting your Buffy rings polished at the little jewelry shop, get your fingers sized, and then shop online. Another good place for the claddagh rings is at ren-faires or craft fairs. These kinds of jewelers will usually sell for much much less than at the mall.