Quickly and roughly. PSK stands for Phase Shift Keying, and all the PSK variants are modulation schemes with various advantages and disadvantages. QPSK is Quadrature Phase Shift Keying and has 4 data bits per symbol. 8PSK has 8 data bits per symbol, so had the capability of more data in a given amount of time. More data per symbol makes it cheaper to transmit large amounts of data, so it's more efficient and cost broadcasters less money to use the 8PSK modulation.
Generally you can't modify a receiver ( unless the manufacturer has built in the upgrade option ) . There are different variations on the 8PSK, which further confuse issues. Dish Network , for example, uses Turbo-8PSK modulation on many of it's transponders.
[edit] For FTA purposes, we generally look for receivers that support DVB-S2, QPSK & 8PSK modulations , mpeg2 and mpeg4 ( h.264) compression . That's pretty much the world standard HD receiver specs.
