The rebalancing process would remain the same for investors with an NRI account as well. However, there are some factors be taken into consideration:
Since rebalancing involves simultaneous buying and selling stocks, if you have multiple smallcases where the same stock is bought in one and sold in the other, you would have unfilled orders since intraday trading isn't permitted for NRIs. You can 'repair' the unfilled order from the Orders page on the next market day.
NRIs can buy/sell most of the stocks, with the exception of some stocks that the RBI has placed restrictions on.
If any of the banned stocks are added during the rebalancing, this would result in an unfilled order, you may 'Archive' the same from the Orders page and complete your smallcase without this stock.
The details of the stocks that can be traded and the stocks with restrictions can be found here.
During rebalancing, ideally, the total amount realised from selling stocks would be utilised to buy the new stocks. If this amount does not suffice, you may need to add additional funds to complete the rebalancing.
However, since remittance of funds takes a few days for NRI accounts, best to ensure you have the funds needed to complete the buy orders, available in your account before you proceed with rebalancing.