You must definitely adjust your play as a tourney progresses. It is a crucial skill for the successful MTT player.
Difficult to sum up briefly, as it`s very situational (and I`m playing in a tourney right now, close to the money and can`t spare that much attention). But, as a rough rule of thumb, you should play tight overall in the early stages, loosening up from late position. As the blinds rise, loosen up and begin stealing.
In addition to the this overall strategy Egon outlined, you should change gears all the time. Every significant change requires adjustements. Some of the events that should prompt you to change gears are:
Your stack became much bigger: Now you can play drawing hands and steal more if the blinds are big.
Your stack became less than 10BB: Look for a good situation to go all-in(first in pot, late position, good hand).
Your stack got smaller to around 20BB: Stop playing drawing hands, don't steal as much.
You doubled up from a short stack to around 15-20BB: Stop going all-in, don't draw, play high cards and big pairs.
You were moved to a new table: Tighten-up to asses the players.
Big stacks were transferred to your left: Tighten up.
You just lost a pot: Tighten up next few hands, especially very next hand since many players assume you are on tilt and will call you more liberally.
Big stack near the bubble: Abuse all small stacks who can make it to the money. Stay away from short stacks who don't have enough to make it or big stacks.
Your cat peed on the keyboard: Use the mouse to select bet sizes.
and so on..... too many to list them all, perhaps others will add to this list
1. The nets time to change your style depends on how the table is playing. If the majority of players are playing tight, be more loose aggressive. If the majority of players ar playing loose, tighten up your play.
2. You shouldn't stick worth the same play because you eventually adopt a table image. You can use this table image to decieve your opponents.
For example, if you raise preflop with AJ or better thorughout the tournament, try raising with mediocre hands. Most of the time, they will fold unless they have something good. This is a good move to use in late position.
I think that there are many things that will and should change your play during a tournament; stack sizes, table image, style, stage in the tourney (bubble, money level), blind level etc.
It's very simple. You play super conservative when your M is 20 or higher. Play tight aggressive when your M is between 10-20. Play aggressive when your M is between 5-10. And play super aggressive when your M is less than 5.