I think the original deck was a bit fussy playing some discs due to the clamper?Praps due to the vertical mounting also.The cd pro tangential deck was/is a complete integrated assembly including decoder,so the signal processing is different.The laser pickup can't make much difference to the sound quality,but the signal processing circuits certainly can.As Graham says,compare the CD50 to the CD5500.The CD50 used Yamaha/Sony technology,the CD5500 Philips/B&O.
I think the upgrade kit included a new clamper also.
The debate over what affects sound quality,and by how much,is wide open to the loony fringe of course,just look at the enduring(and lucrative!)pottiness of the cable enthusiasts!
B&O have always kept a dignified distance from these debates.
I think this cd upgrade is worthwile,but it's only my opinion of course!
Regards,
Nick