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:
En/au/aux = In/to with countries and continents (French Prepositions)
En/Dans = in/to + regions/states/counties (French Prepositions)

I hope it helps!
À bientôt !