I can't say that I can answer your question in its entirety, but who says that Lupercalia is related to Valentine's Day? As far as I'm aware February 14 was simply the feast day of Saint Valentine, with no implications of a celebration of romantic love until the late Middle Ages. Someone can feel free to correct me if I'm wrong on that one, but that's a pretty big gap to fill to get to Lupercalia. And I can't say I see the connection with Lupercalia. Lupercalia was celebrated on February 15, but that doesn't mean much, there's only so many days in a year, festivals between cultures are bound to roughly line up. Lupercalia's rituals have very little to do with the celebration of romantic love that Valentine's Day is supposed to be in the modern west. I mean I don't see the connection between Valentine's Day and the ritual of slaughtering a dog and some goats at the entrance of the Lupercal, smearing the blood with a knife on the foreheads of two boys who were ritually obliged to laugh as it was done, and then having those two boys run around the city smacking people with thongs cut from the sacrificial goat-hides. I mean, I guess there's some sort of possible link there with the fact that Lupercalia is in part a fertility ritual, to ensure bountiful harvests and good offspring, but what that has to do with the romantic love of Valentine's Day I can't say I see. And the fertility ritual aspect of the Lupercalia is hardly the only, or even the most important aspect. The Lupercalia seems to have been primarily a lustration, a ritual associated with purification and aversion of evil, rather than a strict fertility ritual. Coming as it does one month before the start of the archaic new year in March, and one month to the day before the first full moon of the new year (March 15, arguably a more important festival day than the actual old new year) its status as a purifying ritual becomes kind of obvious. I'm not sure what it has to do with Valentine's Day at all

No-- Valentine's Day did not get its start as Lupercalia. /u/XenophonTheAthenian has already addressed the (dis)similarities between the two, but connecting the two is further complicated by the relatively late appearance of a romantic tradition associated with the saint. The first recorded, definitive connection between the feast day and springtime, love, etc. appears in Chaucer's The Parliament of Fowls. If Valentine's Day was in fact connected to Lupercalia, we would need to explain almost one thousand years of silence.

The actual story of St. Valentine(s) is a bit more difficult. There are at least two Valentine's associated with February 14th, though more by that name are reported. We have no contemporary records or accounts of the saints; the first acts of the two do not appear until the sixth or seventh century. The two accounts share similar structures, and are typical of hagiographical writing in the early medieval period. The first Valentine was reportedly a priest in Rome who gave testimony to his faith before an Emperor Claudius (II). Consigned to house arrest in the home of an aristocrat by the name of Asterius, he proceeded to heal the sight of his host's adopted daughter. When the household converted, Claudius had the priest beheaded. The second Valentine was allegedly the bishop of Terni, who healed the crippled son of a pagan scholar named Crato. When Crato and his household convert, the Roman senators order Valentine clubbed and then beheaded. Valentine of Terni is also mentioned in the seventh century Hieronymianum martyrologium and both saints are known to Bede in the eighth. Valentine of Rome also appears in the famous Legenda aurea (1255), the story largely following the earlier iteration. Basically, both saints pop up all over the place throughout medieval Europe, but there is nothing to suggest a romantic association that might connect them with a Roman holiday.

The connections with Lupercalia are largely a product of eighteenth and nineteenth centuries, and though taken for granted well into the twentieth century, it is unlikely that one had anything to do with the other. See:

• Jack Oruch, “St. Valentine, Chaucer, and Spring in February” Speculum, Vol. 56, No. 3 (1981)

For further treatment. It’s a bit dated, but accessible via JSTOR and presents a good case against a Lupercalia connection with St. Valentine.

So, if they have nothing to do with St Valentine's own story, how did the romantic associations appear?