Copyright can exist in many forms, here we are looking at literary and musical copyright, which are classic copyrights. If you have recorded the song before, there will be an additional copyright protecting the contents of that recording.
The requirements are that the right must be recorded. If you haven't written down the lyrics anywhere or haven't written down the music, you are going to struggle to allege the have in fact copied your work. If you haven't done this, then you can simply proceed on a moral but legally weak 'bluff'.
There are further requirements of originality and that the work must be substantial, which have low burdens so I assume these will be satisfied, unless your track consists of one note!
As you created the lyrics, you will be the author of the literary right, but only potentially the co-author of the music. These rights will vest automatically and there is no need for you to register them or something similar. The right will belong to you for 70 years after your death.
I'm not familiar with the rules of co-ownership offhand, but I can tell you that the new lyrics are substantially similar to the old ones and therefore they have copied your right without your consent. You would otherwise be entitled to royalties or similar if you were to give your conditional consent.
Accordingly, you should assert your rights over the lyrics and music to the band members and see what their reaction is. If you just leave it, it may later be construed that your rights have been waived, which is probably not something you want.
I hope that has been of some help, sorry it couldn't be more detailed!
