An informal abbreviation meaning to change a mobile phone model (usually keeping the same carrier and phone number); commonly used in everyday speech.
新しいスマホが出たから、週末に機種変しに行くつもりだ。
Since a new smartphone came out, I'm planning to go change my model this weekend.
機種変したらデータのバックアップを忘れないでください。
Don't forget to back up your data when you change your phone model.
店員に機種変の手続きを頼んだら、30分ほどで終わった。
When I asked the clerk to process the model change, it finished in about 30 minutes.