system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do British replace their coins when the Queen or King dies?

I've searched through articles and found some explanations (unfortunately not an official ones) supposing that the change is gradual. By the wikipedia page I concluded that they start minting new ones with the new monarch immediately after change. 

Does it mean that old coins are still usable for indefinite period of time? Meaning, if I found somewhere in Britain a pound from 1930, I can go and pay with it in shop, and then when it gets to bank they will send it back and destroy?  Is there a period after which the coin with an old monarch becomes physically unusable as a currency? For me it would be a bit counterintuitive as you can often pay with let's say collectible version of a dollar or other currency, even though it's not an official and one The Dollar.
gpt
We don't, coins stay in circulation until they need to be replaced so in times passed you could have a purse with multiple different monarchs on them. At the moment though we only have Lizzy on our coins as in 1971 decimalisation came into effect and all coins were changed so your pound from 1930 could not be used.