Increasing the field type to decimal(15,6) will not have any effect on existing data; 123.4567 will become 123.456700, which is an equivalent value.
Depending on the programs that access the database, this may reflect in text output, i.e. trailing zeroes may appear in textual values retrieved from the database field, but will not affect numeric calculations. My recommendation would be to verify most (or the important) programs that access the database first, if possible accessing a test database with the field type changed, to verify correct output (reports, labels etc). Or, at a stretch, announce half an hour of downtime, switch the field and do some quick verifications on the software side; if there's anything that doesn't look right, change the field back to less decimals (faster than a full backup/restore), which will also not affect data, and work on getting the software patched up - the problem will never be in the database itself unless there are triggers that validate data or something similar.
