Formula to convert first digit of a number to Text

Hi I need help to convert the first digit of a number code to a letter.

Example:

Need to convert 342436 to A42436; and 447623 to B447623 and vice versa,

Can someone help me with a formula please.

Thanks
David

Vishesh's picture

Provide some pattern or logic

Provide some pattern or logic behind it so that some formula or code can be applied