What in a siddur can be changed?

Question

There are many different editions of siddurim with many commonalities but also many differences, see here for discussion on MY. Also different communities use different siddurim to match their traditions and philosophical positions, What Siddur Do Open Orthodox People Use?. But this question about the differences in the prayers in Ashkenasi and Sefardi siddurim inspired this question.

Were a person to find themselves in the position of compiling/editing a new siddur, what are the halachically allowed changes that they can make?

What I am looking for beyond the questions I have found so far is a forward looking answer. That is, not an answer that tells a nice story after the fact to accomodate what happened but rather a forward looking a summary of what would be halachically permissible.

Answer 1

1. Neither individuals or communities are allowed to change their customs, which have the status of vows and are binding upon future generations. Customs relating to "fence" prohibitions can never be undone (as Rebbi Yochanan told the people of Bishan in Pesachim 50b) but other customs can be permitted by hatarat nedarim (YD 214.2, 228.25-26). Rav Moshe (OC 2.21) does not consider a negative minhag binding - you don't say, "Well, we never did that before."

2. The basic structure of the blessings which comprise tefila are matbea sh'tavau Chachamim, the way the Sages put it together. You can't change these parts (Brachot 11a, 40b) - but the definition of "change" requires clarification:

3. The Shulchan Aruch (OC 68.1) holds like Rashi and Rambam that you cannot add anything to the received text, but the Rema holds like Rabbeinu Tam, Rabbeinu Yona, and Rashba that you can add (but not subtract - you must keep the content of the blessing plus the ending). The Mishna Brura, Aruch HaShulchan, and Shulchan Aruch HaRav all agree this is the Ashkenazi custom, and while they note the Arizal did not say piyyutim which were not authored by extremely ancient authorities, his student Rav Chaim Vital said the traditional nusach when leading Rosh Hashana/Yom Kippur (i.e. with all the familiar piyuttim). They mention that this is in reference to the brachot on Shema, but adding to the chazan's repetition is anyhow more lenient because an individual can add his own words to each blessing as appropriate.

4. According to this, the sections of tefila which are clearly from the Sages are: baruch sh'amar/pesukei d'zimra/yishtabach, the blessings before and after Shema, and the amidah. Baruch sh'amar/pesukei d'zimra/yishtabach must be recited first or the blessings cannot be made (OC 52). Shema and its blessings should be recited before amidah in shacharit (OC 58).

5. The Magen Avraham (OC 68) quotes the Arizal there were 12 original nusachot of tefila, one for each tribe, corresponding to 12 gates in Heaven, and they are therefore equally valid. Rav Moshe (OC 2.24) agrees with the Chatam Sofer (I.16) who does not like the claim that nusach Sfarad (or nusach Arizal) is the sha'ar hakolel which anyone can use, and therefore holds it is permitted to switch from nusach Sfarad back to nusach Ashkenaz but not any other way; however the Chatam Sofer's reasoning in Rav Moshe's understanding would seem to indicate that all nusachot we have may really be the same nusach since they share the same fundamental structure, in which case as above it is not clear there is any problem in switching. In any case, hatarat nedarim would seem to be sufficient to switch.

6. Rav Moshe Shternbuch (I.160) says that finding a tefila which inspires you overrides the usual rule of b'rov am hadrat melech ("multitudes of people are the glory of the King") which obligates one to join the largest available minyan, since the essential goal of tefila is kavana ("intent" or "concentration"). It seems obvious that since adding liturgy is permitted, if doing so will increase kavana, it is a good idea.

TL;DR: If you are Ashkenazi, you can add, but not subtract from the core parts of tefila. While major authorities used this power with reserve, it is fundamentally allowed. If you are Sefaradi, you should play it straight and stop causing trouble.