Snapped the top e!

I've done it on my Strat many times when I was too lazy to replace all of them. I had a small burr on the high E string saddle that would prematurely cause the string to break.

I finally took a real good look and found the burr and sanded it down and haven't had a problem since.

I'm sure most people will tell you to change them all though, but I don't think there is any harm in changing just one.

If you notice that it doesn't sound right then go ahead and change them all if they are old.

One new string will have a different tone than the others, because the sound of a string changes over time. If I've had a set of strings on for just a day or two and one breaks, or if one breaks during a gig*, I'll change just that string. But otherwise I'll change the whole set, because one new string is going to really jump out from the others.

If you've never changed the strings, I'd do them all.

And as cnev noted, a small burr can wear out a string. Before you take the old string off, stretch it out over the fretboard and see where it broke. I'll jot that info down - if the next string breaks in the same spot, I'll look at the nut/bridge/saddle at that point through a magnifying glass and see if repairs are in order.

*if I'm doing a gig, I've either got new strings on the guitar I'm using, or I've got a second guitar ready to go.

I'd replace them all.
1) they are of an unknown age.
2) It's good practice to have all the strings in approximately the same condition (as has already been mentioned)
3) buying individual strings is not cheaper than buying a full set (multi-packs are even a better deal). Some brands include a spare of the un-wound strings.

I've bought some new multi-packs and will change the lot - according to Tanglewood customer service the Discovery Range are fitted with strings equivalent to the Custom Light pack from d'Addario.

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If I did have a spare e, d'addario do sell them individually, and I had fitted that, would it never have 'caught up'?

The only difference is the length of time that the string has sat on the guitar, in tune, ( plus I suppose wear from playing)

Strings of different weights/thickness's will never have come from the same 'batch'.

I've noticed on my Classical that the strings take a week or so to settle, which I'd mainly attributed to slack in the bridge knot and wrapping around the tuner peg.

I'm assuming that steel strings need slightly less time to settle as they only have slack at one end, on the tuning peg, as they are held to the bridge by a pin, so no knot. And they naturally wind themselves tighter, just the behaviour of the material.

Nylon (classical) strings take forever to stretch. Steel strings usually are playable the next day.

Like the others, I'd change the entire set. I change my strings about once per month, but then I gig with my guitar.

Notes ♫

In this case, I'd change them all. (Unknown age, unknown quality, good practice.)

But I often change a single string, if the set isn't too old.

I always keep a couple of extra sets on hand, then individual spares of the three lightest strings. (Which are the three that I habitually break.)

The strings aren't "cheaper" one by one, but the end result is cheaper. I change all the strings every couple of months. But by the time that rolls around, I've often changed some of the lighter strings, which I like to break, a couple of times. Using a new set each time would more than double my string costs.

Best,
Ande

Strange.
The only time I've broken a string is when I'm in the process of re-stringing, and I make a mistake.

How do you break that many strings?
Do you play a lot of heavy bends, or lean on your whammy bar a lot?

+1 with kent_eh. I never break 'em either. Not like ande mentioned. Very, very rare do I even break one changing. I even feel as thought I've got a heavy right hand while playing. Maybe even digging too much at times. Not sure how strings break often for some folks.

to the OP, If they've never been changed, I'd do them all. Like has been mentoned, you've no idea how old it is. I think I probably agree with every post so far as far as when to change when one breaks.