Yep
here again...mentions the mirror is different.
Mentions "They also gave it a semi-transparent mirror, so that you can use the LCD for preview and focusing."
So maybe you can buy a 20Da mirror and put it in....but my guess is that canon has thought about this and you will not be able to upload the firmware.
And
here they say it's not semi-transparent. But being no one has posted they did this firmware flash and it worked (in the years both cameras have been around) ...then it's not look'in good..sorry.
It's a pity the sensor pixel size on the 20D and 20Da are ideal for astro imaging.