If you have a 3000psi rated bottle and reg combination from factory , the bottle should show 208/210 bar (cannot remember the exact figure now).
The reg will have a lower rated burst disc 4.5k on the high pressure side, opposed to the 7.5k disc on a bottle and reg combination rated from factory for 4500/5000 psi (310 bar).
You can use a 3000psi rated reg on a 4.5k bottle (not recommended) and fill to 3k.
You can use a 4500psi rated reg on a 3k bottle and fill to 3k.
Although the body of both 3k and 4.5k regs are the same, the internals ie, piston, seat retainer, springs are rated differently.
Some manufacturers put the same rated reg on both 3k and 4.5k bottles. (difference only being the lower gauge reading, and the lower rated burst disc, i would recommend you check with the manufacturer first)
Either way, not good practise to be swoping around different rated components. If you eventually come to sell , it could cause problems.