The word rebase can be unfamiliar, but it is the case of a word that does exactly what it purports. A rebase is a recalculation of a number, series of numbers or of data using a new base or system. For example, say that you are tracking prices. You select a given year's price for something as a base. Later, or as you examine new data, you realise it would be more convenient to use the figure from a different year as a base. To keep your data consistent, you must calculate the rebase, the relative value of prior figure to the new one, so your data is consistent.
Find the intersection of the two data samples in question, or the point at which you want to change over to a new base. For example, if you are tracking something over a period of months -- say, January to June -- and you want to pick up in June using a new base, June is your transition point.
Calculate the relationship between your data collected with the old base, and the data collected with the new base. If your old base is, for instance, 200 and your new base is 50, the new base is 0.25 of the old base, or 50 divided by 200.
Multiply the data figures in the old base -- January to June -- to see what they amount to in the new base. For instance, 300 in the old base would only equal 75 using the rebase.