VPI Jr upgrade

I currently have a VPI Jr and am considering an upgrade to the MK III. What kind of sonic improvement can I expect and can I purchase just the platter and spindle and get most of the same benefits without buying the whole upgrade package ? Thanks
Had a freind do it and said it was worth twice the cost.Not as 1-2-3 as you might want but worth the tine and effort according to him.
I upgraded from the MKIII to the MKIV, and there was a HUGE improvement. You may want to consider that instead, if you can afford it.
The upgrade from Jr. to III or IV is a significant improvement, adding detail, clarity and extension - well worth it, IMHO. I did only part of it, buying the platter, bearing and springs but I made a plinth and armboad out of Corian (you can usually buy a scrap from kitchen remodeling companies). I figured if it was good enough for Well tempered it was worth a try. I produces an amazingly coherent sound across the spectrum, worth checking and a lot cheaper than buying the VPI steel/acrylic subchassis. I have not heard it vs. the VPI but many who have listened to it have been impressed with the sound.