Is it safe to change the length of column (field property in ADMIN /Database manager) if data  is already populated?

Yes, none at all - works fine (you can also do this via SQL DBMgr as well - the datalength/type is picked up from DB - not stored in SLX).

1. as long as you make the field either larger or don't make it small enough to turncate actual data.....

2. in DB Manager as long as you are allowed to change the field (Sectabledefs.UserDef = 'T' for the fieldname/tablename you want to change).

3. If you have remotes and a Dev\staging\production environment where developers never touch production literally EVER.....use a bundle for the field change....