Hi,
What is your source (receiver)?
What size room?
You're not upgrading by getting 3.5" woofers in dinky cabinets.
And you probably need two subs to get good response in a room so don't be so quick to think to sell one.
If you have an actual reciever and not some proprietary connection home-theater-in-a-box ensemble (god I hope not), then get good passive speakers and get away from the shopping/mindset of home-theater-in-a-box kits. They're not good. They're a bunch of tiny tweeters and tiny woofers and a little bookshelf speaker falsely called a "sub woofer" that has to carry all the upper bass, lower mids and probably a lot of the mid range mids as well, since those dinky tweeters can't do much other than spray treble at you. So if you want an upgarde, get real bookshelf speakers and get larger drivers, look for 5.25" to 6.5" drivers and get the largest cabinet you can accommodate for efficiency.
Very best,