Being that we are in the middle of sefirah I decided to talk about sefirah rather than teh parsha.
There is a psak of Rav Moshe Feinstein (Chelek 1 siman 159) that everyone quotes that says you are allowed to switch your sefira minhag every year. People assume this to mean that if in 5768 I kept sefirah from Pesach until Lag B'Omer, then in 5769 I can switch and keep from Rosh Chodesh until Shavuous. However, as I pointed out in my shiur, if you read through the teshuva it is not so simple.
The reasons for the minhag
There are 2 different sources given for why we keep minhagei aveilus during sefirah. The first source is a medrash that says from Pesach until the 33rd/34th day of the omer the talmidim of Rabbi Akiva died. Therefore, we are noheig aveilus during these 33/34 days since these are the days they actually died. The second source is quoted in the name of Tosafos that really the talmidim of Rabbi Akiva died from Pesach until Shavuous. However, they did not die on days which tachanun was not said. This excludes 7 days of Pesach, 6 Shabbosos, and 3 days Rosh Chodesh (2 from Iyar and 1 from Sivan). That leaves 33 days during which the talmidim died. Since they died for 33 days, we choose 33 days during the Omer to follow minhagei aveilus.
The Various Minhagim
There happen to be 6 different minhagim mentioned in Shulchan Aruch and the poskim. However, to keep things simple we will refer to them in 2 groups-1st half and 2nd half. The 1st half minhag holds that one should keep sefirah from Pesach until the 34th day of the Omer. This is the opinion of the Mechabeir and according to this opinion Lag B'Omer is also one of the days in which aveilus applies. This minhag follows the medrash that the talmidim died from Pesach until 34 Omer and therefore we observe aveilus on the days they actually died. The 34th day of the Omer is included but we say miktzas hayom k'kulo and stop once they day begins.
The 2nd half minhag holds one should keep sefirah from Rosh Chodesh (30 Nisan) until 3 Sivan or from 2 Iyar until 5 Sivan. This is the opinion of the Rema in 593:2. This minhag follows Tosafos that the talmidim died from Pesach until Shavuous excluding the days on which tachanun was not said. The 33 days that are picked for aveilus are the days between Rosh Chodesh and Shavuous.
Minhag of Until Lag B'Omer
I purposely left out one important minhag until the end. This is the minhag the Rema mentions in 593:2. Remember that we said that the Mechabeir holds you keep aveilus until day 34 of the Omer. On this the Rema writes that we don't keep until the 34th day but rather we only are noheig aveilus until the 33rd day of the Omer. This is the minhag that many people have of keeping sefirah from Pesach until Lag B'Omer. Really Lag B'Omer is included but we say miktzas hayom k'kulo.
Now comes the big question. What is the reason for this Rema? Is the Rema following the medrash that these are the actual days that the talmidim died , but he argues on the Mechabeir regarding the 34th day of the Omer. Meaning the Mechabeir holds the talmidim died on day 33 of the Omer and on day 34 of the Omer, while the Rema holds they only died up to day 33 but by day 34 they had already stopped dying. This is the opinion of the GRA.
However, the Bach and Pri Megadim understand the Rema differently. They hold that the Rema is working with Tosafos. The talmidim died from Pesach until Shavuous excluding the days on which tachanun was not said, and we need to pick 33 days to observe aveilus. The 33 days that are picked are from Pesach until Lag B'Omer.
The nafka mina between the Gra and the Bach is the basis for Rav Moshe's psak. Rav Moshe writes that you can switch your minhag only if you are switching it to a minhag that follows the same reason. For example, lets say you usually keep Rosh Chodesh until 3 Sivan. You can switch and keep 2 Iyar until Shavuous since the reason behing both these minhagim are the same-both are based on Tosafos. However, one could not switch to the Mechabeir and keep until 34 Omer since that minhag is based on the medrash.
But can a person who usually keeps Rosh Chodesh until 3 Sivan switch to keeping from Pesach until Lag B'Omer (like the Rema). Well that depends. according to the Bach you can since both minhagim are based on Tosafos. Howeevr, according to the GRA you can't because the reason for keeping until Lag B'Omer is the medrash and the reason for keeping after Rosh Chodesh is Tosafos.
Rav Moshe says one should be machmir for the GRA and if one normally holds the 1st half (until Lag B'Omer) unless it is a sha'as hadchak gadol, you should not switch to keeping the 2nd half (after Rosh Chodesh).