Analogue Upgrade Help.

I have a VPI MK4/SME-309 using a Cary SLP98P (moving magnet) preamp. I want to upgrade to a Lyra Helikon (even an SL if that makes sense).

Should I have the Cary converted to MC or should I purchase an AcousTech PH-1, EAR or other comparably priced phono pre-amp.

Do these phono preamp selections preclude the Helikon SL cartridge model selection.

Does my TT/Arm combination preclude the Lyra Helikon or the SL upgrade from a practical perspective.
Your TT and arm are compatible with a Lyra Helikon, if you want one. That is a very nice cartridge. I don't know how good the Cary phono stages are, but most tube units will not have enough gain for the Helikon. I would take a look at good phono step-up device that would allow you to retain the use of your Phono stage in the Cary, since you already have the RIAA section and some gain. A MC Step-up Transformer will allow you to load and step-up the output of the Lyra, and will plug right in to the MM phono section on your Cary. There are a number of makers of step-up trannies, like Denon, Cotter, Shelter, Ortofon, Audio-Technica. Or you could use a "head amp" which does the same thing, only amplifies instead of using a transformer. I see no reason to obsolete your phono stage and go to an outboard one. A step-up tranny or head amp will do. You will want about an extra 30db gain from the step-up device that you get for the Helikon.

From a practical perspective, I generally do not recommend getting a cartridge that is as costly as the TT you put it on, unless you already cannot benefit any further from upgrading in the TT area. In other words, I think that upgrading to a TNT would be more sonically beneficial than getting a Helikon SL on your MkIV. However if you have no intention of upgrading the TT any further, then by all means get whichever cart you want on it.
Save your money and buy a Grado Reference high output version. If you're in no hurry you'll find a great deal on one sooner or later on Audiogon. You will be thrilled with this cartridge. I use a low ouput version (1mV)on my Well Tempered Reference, which replaced a Van den Hul Grasshopper Gold. I use an EAR phono preamp running into an Audible Illusions 3A. If I had the 4mV version that I'm recommending to you I would not even need the EAR as the phono stage on the AI is excellent, just not quite enough gain for 1mV. I imagine the Lyra would be a bit better than the Grado, but as stated by Twl above, your present turntable/arm combo may not do it the justice it deserves. My Well Tempered would, but the Grado is such a good match I don't see the point in spending the extra money. The Grados are that good!
TWL and Tswhitsel - Thanks for the kind help and advice. I am planning to upgrade to a TNT or equivalent TT in a couple years but not immediately (ongoing kitchen/living room renovations preclude the added expense). I planned instead to upgrade the cartridge first, but I was under the false impression, based on reviewer/reader comments in Stereophile and TAS, that MC was the only way to go. I am very encouraged by Tswhitsel suggestion of the Grado Reference. I considered this cartridge but did not know what to expect. I use a Clearaudio Beta Aurum cartridge today and used a Grado Platinum last year.
I certainly agree with the recommendation of the Grado Statement Reference in a high output model. I also have a MK4 TT with a classic Grado Signature arm; this cartridge sounds fantastic with this combo. Surface noise is so extremely low; it's just amazing. I have my VTA set such that the cartridge body is almost exactly parallel to the record, maybe just slightly negative, depending upon variables.
Thanks Bob_bundus. I appreciate the input and the help.
Thanks Bob_bundus. I appreciate the input and the help.