I know there has to be a simple solution, but I can't find it

Sheet1 has values in column A. Col A is formatted as text and contains
either an
ID 'number' or the words NOTES

Sheet2 cells are linked to the cells in Sheet1.

If an ID in Sheet1 is changed, then Sheet2 cell needs to change, but if
Sheet1 cell changes to the word NOTES, then Sheet2 needs to retain the
original ID.

For example
Sheet1, Col A Sheet2, Col A
Row 2 1234 1234
Row 3 5678 5678
Row 4 9999 9999

The value in Sheet1, row2 is changed to 3838, then Sheet2, row2 should be
3838.
The value in Sheet1, row3 is changed to NOTES; Sheet2, Row3 should remain 5678

I tried putting a formula in Sheet2 cell:
=IF(Sheet1!A2<"NOTES",Sheet1A2) (which gives me a circular reference)

Your help would really be appreciated!