Surcharge (eg. New currency):
Of course surcharge may not be only a new currency. Often, 'usually' I would say, it was because the PO had run out of a certain value, and as a 'stop-gap measure' overprinted a value that they had ample stocks of.
Imagine you're a colonial postmaster. You're out of 2 cent stamps, which pay the local letter rate. To get more would mean writing a letter ordering them from the printer in Europe--several weeks for your letter to sail to the receiver, then weeks for the printing to be done, then weeks for the order to be delivered. Solution, take stamps with a face value of 4cents, or 8, or 24, or whatever you have a lot of and don't get used a lot, and have a local printer with type-setting equipment overprint them.
The 19th century issues of Thailand are a complicated 'mess', as this was done for years, and the major varieties in the catalogue alone cite various fonts (Antique, Roman...). Then there are plate-able varieties (short bars, broken characters...).
Related to overprints are surcharges
. That is when a stamp has a 'tax' added to it. This was often done for disaster relief efforts, it was an 'immediate' way of creating a fundraising semi-postal. Sometimes the tax was mandatory, and the surcharge was added because after all, a stamp is a receipt and proof of payment, so the PO has to show that the price on the stamp is the price you pay.