I am trying to change data from:

A 1 2 3
B 1 2
C 1 2 3 4

(presume that is 3 rows and 5 columns)

to

A 1
A 2
A 3
B 1
B 2
C 1
C 2
C 3
C 4

Can anyone help with formula, or hints on how to get there?

Here's a way that might get things started. It's not elegant, but it lets
you see both the original data and the reorganized data at the same time,
and lets you edit the original data with the reorganized data tracking
it.

Begin with the data in columns F:R (insert blank columns if needed). The
result will be in A:B. Helper columns are C:E (doncha love-'em?).

In E1 put 0 (zero).

In E2 put
=E1+COUNTA(G1:R1)
and copy down to one row past the end of column F (at least).

In A1 put
=IF(ROW()MAX(E:E),"",VLOOKUP(ROW()-1,E:G,2))
and copy down for the maximum number of rows you could need in the
reorganized data.

In C1 put 0 (zero).

In C2 put
=IF(A1=A2,C1,C1+COUNTIF(E:E,ROW()-1))
and copy down as far as column A goes. (The COUNTIF is for the case when
a row has columns G:R all empty.)

In D1 put 1 (one).

In D2 put
=IF(A2=A1,D1+1,1)
and copy down as far as column A goes.

In B1 put
=IF(A1="","",OFFSET($F$1,C1,D1))
and copy down as far as column A goes.

Hide columns (or use more than one worksheet) if you want less clutter.

Limitation: this doesn't work if (for example) H4 contains a value but H3
is empty.