kikutano wrote:So I've to "unload" and "load" 512 tiles at time?
It really depends on the mapper. CNROM, one of the simplest mappers out there, allows you to have 4 8KB pages in total, and you can only swap all 512 tiles at once.
On the other extreme there are mappers like the MMC3, which divides the pattern tables into slots of 1 and 2KB, so you can swap smaller chunks at a time.
There's yet another option for managing graphics, which's CHR-RAM. Using RAM for your tiles means the cartridge doesn't have a separate chip where CHR data is stored, instead it has a RAM chip. The CHR data is then stored in the PRG chip, in whatever format the programmer wants, even compressed, and has to be manually copied to the CHR-RAM chip at run time. In this case, you have total freedom to replace tiles as you wish, but unlike with CHR-ROM, the switch isn't instantaneous, each byte has to be explicitly transferred by the CPU, consuming processing time.