Replacing an image sequence in Motion with a longer version of the same image sequence

The easiest thing to do is simply import the second sequence by itself — don't try to *replace* the original. You can just delete the original, or if you need it, turn off its visibility (the checkmark on the left side of its layer.)

Be aware that if your original project is only 100 frames long, the 200 frame sequence will not retime/resize your project — you will have to do that manually. That's easy enough to do by clicking the small clock icon on the right side of the time clock. The "hands" will change from blue to white. You can select Frames from the downward pointing caret and type in the number of frames you want the project to be. Click the clock again to go back to a "play clock".

If you reset the time length, you will probably need to reset layer lengths as well (in most cases). Place the playhead on the last frame of your project and go through your layers list and type 'O' to set new Out Points.

HTH