This is a common occurence and no problem whatsoever. The only downside is exactly what Circle has said, you will only obtain the new lower grant, unless it is solar that you are installing. Then, you will have meet new criteria of Part L 2008 Building Regs by producing 10kWh/m2/annum from a renewable source, i.e. solar. To do this, you will be installing a far larger system than anticipated or that you will require.
The grant approval process is applied for on-line and you should receive instant confirmation of approval.