Siddur Tefila Yeshara (the Berdichever Siddur) has, in the second blessing of the Shemoneh Esreh, ורב להושיע - "v'rav l'hoshea", "and abundant in deliverance". Literally every other siddur I've ever seen omits the initial vav for "abundant in deliverance". Is this attested to in any other siddur? What is the basis for the change?
Siddur Ezor Eliyahu claims that ורב was the nusach in מקצת סידורי פולין -- some of the siddurim in Poland, but not found at all in the western Ashkenazic rite. (He doesn't discuss Sefardim.) Berdychiv, while not in modern Poland, is certainly not a western Ashkenazic community so I guess that it is likely just the nusach they have always had and there was no proactive change.
As to why the competing nusach dominates in modern siddurim, I cannot say for sure, but the added vav-hachibbur strikes me as odd when so many other items on that list do not have one unless obviously connected (eg מכלכל חיים then מחיה מתים then סומך נופלים but then ורופא חולים ומתיר אסורים). So I speculate that those without the added vav view רב להושיע as a separate item from מחיה מתים אתה, a read which seems more straightforward to me.
Seder Avodat Yisrael by Rabbi Seligman Baer comments that the nusach with the Vav is found in many old siddurim, while the now-prevalent nusach without the Vav is found in the Tur and in the Abudarham. He adds that he considers the nusach without the Vav to be the "ikkar" as it reflects Isaiah 63:1 (as Yoel pointed out in the comments). Seder Avodat Yisrael was widely respected as authoritative and printers of later siddurim were influenced by its determinations. Link to the page in Avodat Yisrael: http://www.hebrewbooks.org/pdfpager.aspx?req=42807&st=&pgnum=108