Mdthbs

Can a PC's alignment be forcibly changed?

I've been skimming through the classes on PFSRD, and noticed a lot of them have alignment restrictions. Many state that if their alignment changes, they lose X, Y, and Z, or just stop gaining class levels, period.

Since that's the case and a number of classes I'm interested in have alignment restrictions, I wanted to know if that's possible so I don't end up losing my hard-earned levels later down the line.

Yes, albeit not irreversibly

There are effects in the game which can cause changes to a PC's alignment against their will - the most well known of which must undoubtedly be the helm of opposite alignment:

When placed upon the head, this item’s curse immediately takes effect (Will DC 15 negates). On a failed save, the alignment of the wearer is radically altered to an alignment as different as possible from the former alignment—good to evil, chaotic to lawful, neutral to some extreme commitment (LE, LG, CE, or CG). [...]

Only a wish or a miracle can restore a character’s former alignment, and the affected individual does not make any attempt to return to the former alignment. In fact, he views the prospect with horror and avoids it in any way possible. If a character of a class with an alignment requirement is affected, an atonement spell is needed as well if the curse is to be obliterated. When a helm of opposite alignment has functioned once, it loses its magical properties.

Except when specifically stated otherwise (such as by the helm above), the atonement spell is a panacea for most involuntary alignment changes that might affect a character, as it has the capability to unconditionally reverse magical alignment changes and restore class features lost due to alignment restrictions (so long as the character has returned to a compatible alignment). Forced alignment change is also a relatively rare in-game mechanic, so it's not something you should probably worry about much on that basis alone.

What's more common will be your GM enforcing an alignment change on a character based on the way they act in-character, which could be contentious if you and your GM don't have the same views on the meaning of alignment. If you play a character class that has alignment restrictions, I would strongly suggest talking to your GM about what you both think the relevant alignment means, so that you can come to an agreement (or at least you know how your GM intends to rule if it comes up) and you aren't setting yourself up for an unpleasant shock later on.

Yes. Alignment can be changed involuntarily

It can be from magical effects, choice, or behaving in a method contradictory to your alignment. From the GMG on Changing Alignment:

Changing Alignment

While alignment is often a static trait, options and effects exist that might cause it to change, and players might seek to change their PCs’ alignments for a variety of reasons.

Voluntarily: Aside from merely having misunderstood what a specific alignment means, PCs might seek to change their alignment in light of game events or to qualify for some alignment-related goal. How this change takes shape should be determined by the player and GM. Often, some quest, trauma, rehabilitation, or other life-changing event triggers the alignment change. Players should be sure of their decision, as changing alignment should be the result of an extraordinary effort, not a whim, and a PC with a shifting personality risks losing definition as a character and might begin to seem like he’s trying to exploit the rules.

Involuntarily: All manner of events might lead a character to have an involuntary alignment shift. Some are truly involuntary, as some force overrides or corrupts the PC’s personality. The GM should work with the player in such cases, perhaps making an unnaturally compromised PC a confederate in an ongoing story. Stepping out of the norm and playing her own character in a contradictory fashion can be fun for a player and delightfully unnerving for the rest of a group.

Characters also risk having their alignment changed if they continually act in accordance with an alignment other than the one they chose. For many characters, this matters little, but in the case of characters bound to a specific alignment for rules-related purposes, an alignment change might mean having to reimagine their entire character. Instead of springing this on a player, make it clear when an action risks violating an alignment-related requirement. Sometimes this will be seen as the voice of the character’s conscience, and allow the player to refrain from the action or suitably justify it to themselves (and you) to bring it into accordance with their values.