I have many pages that I want to change the content of. I am not going to change all the meta tags now, I dont have the time. Will the search engines just disregard the meta tags when they no longer match the content or will I get a penalty so that they will not index the page at all ?
In my experience Google almost doesn't even look at the Meta description and keywords, make sure your title for every page is different (very important). Yahoo and MSN look a bit more at the Meta Dscr and kwords but I don't think it will hurt you, your pages just wont get ranked as well.
Ok, but stuffing keywords (that will not appear in the content) in the meta tags has been a well known spam method long time ago. Maybe they still are on the alert for that. And beside that, I want to rank well : )
No offense. But do not lazy when it comes to seo, if you want your site to well in the serps then take the time that is needed to do so. It should not take no more than hour or 2 get things set up the way they should be. Besides it good practise to do so.
Aside from anything else, the description is what appears with your page in the serp returns so you want them to be relevant to content or you'll get badly targetted traffic.
If you are ranking well, i would suggest not to play with title tags of your pages as it can leave you with very unpleasent changes in your SERPs.