In short, nothing. SIP rebalancing is only for future SIP installments. It only changes the funds in which your future SIPs go and does not involve any changes to your existing portfolio. 

For example, if your SIP were in funds A, B & C and our latest recommended funds are X, Y & Z, then SIP rebalancing will simply modify your *(future) SIP to funds X, Y & Z. 

To redeem money (from existing funds) and reinvest it in the new recommended funds, you can do portfolio rebalancing.