Whether one has had braces/Invisalign or not, teeth are continually moving to some degree and it may not always be percetible. Your teeth probably don't look the same today as they did 5, 10, or 15 years ago. Continual biting, chewing, and other functional forces help to cause these alignment changes in tooth alignment over time.
That being said, certain teeth alignment problems need to be retained longer than others. For example, open bites, gaps between teeth, twisted or rotated teeth (especially upper and lower front teeth) have a high tendency to relapse or move back to their original position. Therefore, retention is more critical in these kinds of cases.
In your case specifically, it appears you have a diastema or a gap between between your upper front teeth. This problem is relatively easy to correct but very difficult to retain. Diatemas like these are usually associated with thin or thick band of tissue ("meat") comming from the upper lip and attaching to the area adjacent to gap between the teeth (pull your lip out, away from your lip to check). If present, I usually recommend a frenectomy to be performed which stabilizes the space closure in this area. Ask your provider about this option. Spaces between teeth, once closed, have a strong tendency to want to open again.
For retention, A.) fixed retention is an option. With this, a small wire is bonded or glued to the back of your front teeth.
Pros: You don't have to remember to wear a retainer. If this option is chosen I would strongly recommend an additional Invisalign-like retainer to fit over the fixed retainer as insurance, in case of breakage.
Cons: 1) The wire can become un-glued or break (it is not always apparent when this happens) and your teeth will move or space opens again (requiring additional treatment and additional costs. 2) In some individuals, can be uncomfortable to the tongue. 3) Make it more difficult to floss and clean.
B.) Removable Retainers - There are several types of removable retainers. I like the clear "slip-cover" retainers which look like the Invisalign trays but made of a stronger longer lasting material. I would not recommend wearing your last Invisalign aligner as a retainer for an extended period of time. These are made of a material that are only meant to last for a few weeks before they start to break down and crack.
Pros: 1) Adequate hygiene can be maintained. 2) If you loose it, you can immediately get a new one without teeth moving. 3) And best of all, wear time gets less and less as time goes by - if worn as recommended, you will eventually get to the point that frequency of wear will be minimal i.e. once a week at nighttime.
Cons: 1) You must wear it as instructed and keep up with it. 2) Easy to loose for some people. 3) Full time wear schedule initially which will decrease with time (see your provider for schedule)
P.S. Just because your BF had "bad" teeth doesn't mean you have to...
