Learn English – Word for changing or shortening or abbreviating a given name

I came to this website when I was looking for a particular word – but did not use a thesaurus because I could not think of a good synonym.

Instead I searched for

Word for changing or shortening or abbreviating a given name

After amending the original search to try to get what I am seeking & so far what is here is the nearest but I KNOW there is a word for exactly that which I seek.

THIS is getting to be a long entry and unlikely to attract any, let alone many readers.

I am writing in England and am 66 years old – the word 'fanny' in my era is slang for the most intimate part of a woman's body, yet means other things in other versions of English, and is indeed used as a woman's nickname in England on some occasions. So what I am after is something about renaming akin to a nickname – I had hoped it will come back to me but it has not yet!

All this was prompted by the sight of a product supplied in a can – I am not sure where from – that contains what I know as tuna fish but is called 'Fanny' and therefore if it were seen in a British provision store might produce a smirk or embarrassed giggles because some, such as me, would consider it rude – I have said too much.

My wife could not help, she compared it to 'Betty' as an alternative to Elizabeth – but I do not think that is quite the same – as Betty – to me seems – a definite abbreviation of Elizabeth.

I may have posted this in the wrong place – I am happy for it to be repositioned perhaps with a more accurate subject line – BUT I just want to recall that blessed word and improve my grammar!

I imagine my sentence might be –

In the family she is known as Fanny
because that is how we have XXXXXXXXXXX her formal name of
Ermintrude/Glorianna/(or whatever)