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.
There are no charges or taxes involved in SIP rebalancing. To know why it's important to rebalance your SIPs, click here.
To redeem money (from existing funds) and reinvest it in the new recommended funds, you can do portfolio rebalancing.
We also have YouTube videos available for all the common FAQs on SIP and Portfolio Rebalancing. This will hopefully make it simple to understand the entire process and help clarify any doubts that you may have.