Find and Replace Substring

Here is an interesting problem that I am having a hard time coming up with a vba solution for. I have a set of strings similar to this:

=SERIES(Month!$L$6,Month!$A$7:$A$19,Month!$L$7:$L$19,1)

I would like to replace the "19" programatically. The tricky part is that I wont know that the number is 19. IE. I cant just simply search for 19 and replace it because there are many of these strings all with differing values.

I need a solution that does something along the lines of this:
1. Look for second comma in string
2. Replace everything to the left of this comma and to the right of the first "$" with some other number
3. Repeat for the 3rd comma in the string.

If anyone could provide a function, or guidance, that would be greatly appreciated.