Near the bottom of Nozir 48a, the Gemoro presents a possible Kal VoChomer from Kohen Godol to Nozir, but shows that there is a Pircha (re. Korbon if becomes Tomei).
However, a few lines later, the Gemoro uses a similar Kal VoChomer from Kohen Hedyot to Nozir, and this seems to remain. Why could we not use the same Pircha as before?
In the first case the Gemara is trying to learn a Kula, i.e. the Heter to be Metamai to a Mes Mitzvah. Therefore it asks that Nazir is more stringent. In the second case the Gemara is learning a Chumra from Kohen Hedyot, i.e. that the Nazir may not be Metamai to other Mesim (to dispel the P'shat that he may not be Metamai only to relatives). Therefore, the Kal v'Chomer remains.
D. Zupnik