I changed mine from 1600 to 5000 with no problems.

Seems it has changed;

Change to engine and cylinder capacity (engine size)
DVLA will need written evidence of any changes to engine number and cylinder capacity (cc). Written evidence can be:

• a receipt for the replacement engine
• written evidence from the manufacturer
• an inspection report provided for insurance purposes
• written confirmation on headed paper from a garage if the change in engine size took place before you bought the vehicle

.