"Mass Effect 2" only allows players to pick a class during character creation. This doesn't mean you're out of luck importing an existing character, however; you'll get the same character creation and class options at the beginning. Once you've confirmed your class, you can't change it naturally for the rest of the game.
Changing an Imported Character
After the prologue, players have the opportunity to modify Shepard's face and class whether an existing one was imported or not. Depending on Shepard's existing class, you might not have a choice; specializations were changed between games. For example, while a Vanguard could previously be a Nemesis, this specialization is exclusive to Adepts in "Mass Effect 2."
Modifying a Save File
Once you pick a class, you normally can't undo it. You can modify your game save with a third-party editor. It's not recommended to try this without some technical experience, however; changing a class requires replacing one specific number with another in the raw files of the game. Tutorials are available to help simplify the process. See the link in the Resources section for some examples.
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