It's a little bit more complicated - you need to apply to the RHS to have your existing (NQF) qualification taken into account for the new Diploma.
If you go to the actual specification for the two level 2 qualifications which are the equivalent if your RHS qual and read through all the prelims to page 9 'Exemptions':
The ‘Regulatory arrangements for the Qualifications and Credit Framework’ allow
exemptions to be granted for units based upon certificated achievement in other
qualifications. RHS Qualifications will grant exemption from specified units in RHS QCF qualifications where a candidate has passed certain components of RHS qualifications in the National Qualifications Framework (current RHS qualifications).
All requests for exemption will be reviewed on a case by case basis. Applications must be made
to RHS Qualifications and an administration fee will be charged. Candidates will need to have obtained 50% of the available marks for the NQF component (within the Paper/Module) in order to obtain exemption from the QCF unit.
All exemptions aretreated as equivalent to a pass. No certificate or credit is awarded for units for which exception has been granted.'
All of the Plant Growth Certificate can be exempted; however the first unit of 'Garden Planning' contains some material that wasn't in the 'old' certificate, so to carry your level 2 certificate forward to gain the diploma you need to sit that unit, plus apply to the RHS for exemption in all the other units.
Hope this helps,