Yeah, you can put a new gasket on over the old one, but it's more comfortable and neater to pull all the old one off. You can also use a coffee can, but it's a *lot* easier (and you'll get do a better job) to make or borrow a neck gasket "kit". The "kit" has a couple flat templates that you use to clamp the gasket down with spring clamps. It's still early - you should consider just eating the $40 or so for a new neck gasket, put it on now, and forget about it for the season.
Have a look at the picture at:
Dry suit neck gaskets, drysuit seals and dry-socks, dry top neck gasket and wrist seals, drysuit gasket and seal replacement tools and repairs
The tool really makes a difference. Also, when it's time to replace your wrist gaskets, don't bother trying to find a "correct" sized bottle (champagne bottles fit best). When you use any tapered bottle, the damn gaskets will slip and it's a real hassle (in my experience) to get them to set up correctly. As a much better alternative, just cut a piece of minicell the 'right' size (a snug fit inside your jacket sleeve, which will be about 18" in diameter - gaskets are now one standard size at the sleeve end) and cover this with clear packing tape. Use this rather than a bottle and the gasket will be less inclined to slip off. Put a couple rubber bands over the top of the glued gasket to ensure a watertight and complete seal.