Access Combo Box question

[Mr Smith]

I've inherited an Access Database, and am struggling to make what I
thought would be a simple change.

There is one item on the form defined as a "Combo Box". It expects
values A, B or C at the moment, but my boss has asked if I can extend
it to include D.

So.... I opened the form in Design View, and saw that the field had a
"Lookup Row Source" of "A";"B";"C".

Very smugly I added ;"D" to the string, only to find it just will not
take effect, no matter what I close and re-open. Thwarted, or what?

I've had a dabble on-line and seen similar posts on this subject, but
the solutions all seem to involved embedding code somewhere (which
would be a "first" for me!)

Is there really no way I can do this just by altering a few field
definitions? Alternatively, if I changed it to a "List Box" or
something (don't really know the difference, if I'm honest) would that
solve things?

Thanks for any suggestions!