If its locked
If it has been accepted.
Then it's most likely being worked on or in the queue , it might be a little change that won't have much effect or it might be a big thing that staff will work on when they feel its worth it. It could also be that the thing has been added and removed. If you think the suggestion should still be added then make a post about it.
If it has been declined
The reason is most likely permanent and won't change but in the rare case you believe the situation has changed or the reason doesn't hold up anymore then remake the suggestion.
If the post its not locked
Why wouldn't the post be locked ?
Suggestions need to be discussed before a decision is made to think through if there is any issues on it and if the suggestion is a lot of work or a small thing that will have little impact then they might leave it for when they have more time rather than committing to it straight away.
Comment on the older post , there is no reason to remake the post because in most cases you will get the same feedback. Commenting on an older post will cause it to be displayed at the top of the forums and people will be reminded on it and can express how much they still want if they want to.