I'm getting a new STi grill for my '03 WRX soon. I wanna remove the blue subaru badge and replace it with a red "i" badge. What's the best way to take the old one off and install the new one so it stays on?

Badge removal is similar to the method used for your previous question about removing S-U-B-A-R-U lettering from trunk lid.

Use hair dryer to gently heat front badge emblem. Use screwdriver to gently pry up an edge, then use fishing line to slice double-sided tape securing the badge. Remove old tape from grill area. Use new tape to affix new emblem.

For other threads on the subject, use forum Search with keywords 'remove badge'.

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You don't have to use a screwdriver to peel them off if you blow dry it for long enough. You can actually just pick a corner and peel them off and it won't scratch. I would be afraid to have a screwdriver that close to my car, but that's just me. Wingless, I'm not knocking your method, just adding to it.