If it has been too long, there is a good chance that a large percentage of your list is going to bounce due to non-existent email addresses. I always let clients know that the "very first" email campaign to a list that hasn't been marketed to before is going to produce a lot of "unwanted" results on that first send. But, that is when you get to clean up your list! :)
I typically recommend Constant Contact for small to medium sized companies who don't have an email campaign management program internally, some do, it may have been part of the ecommerce package or it was built to suit.
Any reputable third party email management company is going to require that your subscribers be at least "opt-in". If they are not, you stand a chance of having your account suspended, so be very careful in how you market that first announcement.
I also recommend that email campaigns like this be sent from a secondary domain and/or the third party domain which you will end up doing in most instances anyway, sending from the third party domain. This protects your "asset"! You do not want to send from the primary domain as that first group may be your worst enemy. There will be Spam Reports, Opt-Outs, etc. If you cross a certain threshold, your account may be put on hold while that companies QC team conducts a review.