Character separation in text

Hi!
Sorry if this has been already discussed, I haven't been able to find a solution via search.

I'm working on a simple logo animation in which I have a string of characters. I have animated them so that they change value over time (i.e. a random letter shows for each character every frame), till the correct one shows up to form the logo... pretty straightforward stuff.

Now, the tricky bit is that my string of characters (a single long word, actually) has a separation in-between characters which is dictated by the final word. When the character values change, the space between them does not, thus messing things up quite a bit.
As an example, if I have a string which reads S-I-G-N-A-L, the "I" character is quite close to the next one; when its value changes to, say, a "W" (which is obviously much wider), the space does not adapt, and the "W" overlaps the "G", which doesn't look quite right.
The most obvious solution would be using a fixed-width font, but I cannot because I have to stick with the logo typography.

I have fiddled with all kind of text parameters to no avail, and I'm not that much into expressions to know if it can be fixed this way.

Any help would be much appreciated.
Thanks a lot in advance.
Alex.