Use record number of subform to assign letters

I have a subform in which each record has a letter starting from A and 
preceding alphabetically. I want to make it so that if one record in the 
subform is deleted, the letters will automatically change to accomodate it

with no gaps.

For example

ID       Record Number(in subform)      Letter
34                   1                                        A
35                   2                                        B <----- 
delete this record
36                   3                                        C   

then goes to

ID         Record Number(in subform)     Letter
34                     1                                      A
36                     2                                      B   <-------

auto changed assigned letter

Any help given would be great!

Thanks