Re-basing essentially means changing the reference prices that are used to value output. So if the reference prices are outdated, then your current GDP will be under-valued. Also, the price of certain outputs may have increased dis-proportionately, and re-basing ensures that the relative value of each output is correctly taken into account. So expect a significant jump in the GDP once re-basing takes place.
However, you also mentioned growth rate. Here, it gets tricky. While re-basing ensures more accurate valuation of current output, it also leads to distortion of rapid current growth. Suppose Pakistan’s economy is growing at 6% based on current GDP, once re-basing takes place, the base itself will be higher. As a result, the growth rate will be reported as being lesser, maybe 5%, whereas in essence it is growing at 6% only. There is a proposed solution to this issue by which you calculate GDP at both old and new prices and then average out the previous years, depending on how far away from each year we are.
The reason for India’s higher growth rate was not re-basing. Re-basing increased the total GDP, but not the growth rate. The growth rate reported was higher because of increase in the residuary categories, basically greater inclusion of informal sector for which precise statistics are difficult to get. It definitely is more of a guesstimate than accurate figure.