At this stage (24 Feb 2018) SARS have not announced what changes there will be to the VAT201, but it is quite likely they will want to see the figures split between those charged at 14% and those charged at 15%. In Omni this is not a problem. On the VAT return, click the relevant button to drill down to the VAT transactions making up each figure. Make sure your grid is grouped by VAT Rate (not VAT Code). The sub-totals on the grid for each rate will then give you the relevant figures.
Update: 2 May 2018
SARS are not making changes to their return to cater for the mixed rates in the March-April 2018 period. Since their return needs the VAT to match the Inclusive value in Box 1, they have asked that you put the 14% values into the boxes usually used for credit notes and other adjustments.
Note: this only applies to the 2018 04 VAT return, you should not have any 14% invoices on the returns after this. Only credit notes may be at 14%, and Omni automatically puts credit notes into the adjustment box, so those going forward will not be a problem.
How do I see how much of box 1 was 14%?
When preparing your VAT return, double-click the box to drill down to the transactions.
Group by VAT Rate (NB. use VAT Rate, not Vat Code).
Note the VAT and Inclusive totals.
Make a note in your VAT memo as to how much you manually transferred to the adjustments.