I've been told that if the town/city begins with a vowel the 'à' changes to 'en'. Is that right?

Bonjour Anne !

I'm afraid you're mixing two different lessons :)

To say "in/to" + city, you always use "à", regardless of spelling.
But it's a different matter when it comes to countries, or regions/states...
Here are links to our related lessons:
https://progress.lawlessfrench.com/revision/grammar/use-en-with-feminine-countries-and-aux-with-masculine-countries-to-say-in-or-to-prepositions
https://progress.lawlessfrench.com/revision/grammar/prepositions-with-regions-states-counties

I hope it helps!
À bientôt !