When I suggest that a talk should always be prepared with a specific audience in mind, people often misunderstand this to mean that they should start over from scratch every time - and are, understandably, horrified by the seeming amount of extra work that this implies.
But that's not what I meant.
Preparing an existing talk for a new audience means that you should take a good look at your talk from the perspective of that new audience. Is everything in there really of interest to them? Maybe some aspects are more important to this audience than they were for the last audience.
So it is perfectly okay to give the same talk to another audience again, assuming that you did your homework and checked your new audience's interests and background.
The absolute minimum you should do, however, is to update your welcome slide (and possibly other slides) so that it mentions the new event. A welcome slide that proudly displays the name of another past event will send the wrong signal - your audience will think they're seeing something old that wasn't really meant for them. So even if it is the exact same presentation you gave before, at the very least update that first slide!
What can work, by the way, is giving a "sneak preview" of a talk for an upcoming (possibly bigger) event. As long as there is at least an overlap of interest between your preview audience and the real one, this is probably the only situation where you can get away with giving a talk to what is not really your target audience. I often do this with local groups of people that I know well: I get to test my presentation in front of a friendly audience and they appreciate the chance to see a presentation that they may not otherwise be able to attend (since it's to be given at an event they aren't able to attend). This "win-win situation" makes the possible mismatch in focus acceptable.
A side effect of refocussing your presentation, i.e. making the necessary and usually small adjustments for every new audience, is that sometimes new topics for presentations emerge.
For example, a problem I have at work is that of visualising the status of a complex system. Initially, this was just one of many aspects in a talk about Continuous Integration that I prepared for a conference. It was still present in later iterations of that talk, given to different audiences. Later on, I needed a topic for a 15-minute talk, so I focussed on this specific problem and also expanded it somewhat, to include feedback I had already received during the previous incarnations. For the next iteration, a 5-minute lightning talk, I had to strip it down again and really focus on the core of the problem. I have since found that, while this a very specific problem I'm having in my specific work environment, the 5-minute version is now suitable to explain the problem even to audiences which are not familiar with my line of work.
Reusing an existing presentation is perfectly okay but only if you make sure that your new audience has the same interests and needs as the previous one. Don't forget to update your welcome slide!
If you do refocus and refine the presentation for a new audience, you will notice that, over time, aspects may emerge that require their own presentation - which will keep you "in business" as a speaker and ensure that you're not boring your audience with the same topic all the time.
Please email me for details.