No you cant charge the previous tax rates.
It describes time of supply of services Sub section no 03 & 04 of section no 04 of Value Added Tax act no 14 of 2002.
it read as follows,
(3)The supply of services shall be deemed to have taken place, at the time, of the occurrence of any of the following whichever, occurs earlier: –
(a) the service was performed; or
(b) a payment is received for the services rendered or for future services; or
(c) a payment is due for the services rendered or for future services; or
(d) an invoice is issued in respect of the services rendered
(4) Notwithstanding the provisions of paragraph (a) of subsection (3), where an invoice is issued in respect of services supplied, within ten days from the date of performance of such service, the time of supply of such service shall be deemed to be the time at which the invoice was issued.