You should be able to add the full name for the initial given in a person's name. Some of the WWI records only record the first name, initial and surname. The only way to add the full name for an initial is to add this into the "Served under Name". But this does not update the name which appears in the index. EG. The full of Henry C Barron is Henry Charles Barron.
Hi there - you can definitely do this without using 'served as'.
Once you've added evidence showing their full name, click on that evidence to add facts.
In the 'add facts' box, select 'all names', and as well as the option to fill in 'served under', you should also see the option to 'improve' the name already listed.