I've got some festoons for a theatre show but the theatre is quite old and I was told they need to have 15amp round pin plugs. Can I just get some ordinary festoons and swap the plug into a 15amp round pin plug? And is changing them a very tricky undertaking? Or are there any adapters for that for those sort of plugs? Any answers would help thank you very much Lia
Yes, you either change out the plug or get an adapter. Adapters from round pin (more commonly known as stage pin) to whatever the common mains socket where you live will be easily available at any theatrical supply store near you, or online. If you'd like to make it permanent, instead just buy a stage pin plug and change out your current cord end -- if you've ever changed a plug before, it'll be the same straightforward process.
Here's a link to an adapter that will work with standard North American plugs: https://www.lighttheatrics.com/lex-edison-female-to-stage-pin-male/
Also note that use of these plugs is not because the theatre is 'quite old' -- this style is still used in new theatres today. The reason they use a different plug style and make you use an adapter is because all of these circuits are on dimmers, which are really only meant for incandescent lamps, and don't play nicely with other types of electronics, so the incompatible plugs serves as a reminder that it isn't a general purpose circuit that you can plug just anything into.
Nate Strickland's answer is correct for the USA.
If you are in the UK, you'd need a similar adapter
15 amp round pin plugs are used for stage work in the UK for the additional reason that our 13 amp standard power plugs have fuses in them. A blown fuse on a lighting grid is difficult to change quickly, so unfused plugs are preferred. In schools the smaller 5 amp round pin plugs were often used.