Given that for a period not too long ago six of the nine justices were Republican appointees
I think you mean seven of nine: John Roberts (by George W. Bush); John Paul Stevens (Gerald Ford); Antonin Scalia (Ronald Reagan); Anthony Kennedy (Ronald Reagan); David Souter (George H. W. Bush); Clarence Thomas (George H. W. Bush); Samuel Alito (George W. Bush). Note that Roberts and Alito replaced William Rehnquist (Richard Nixon) and O'Connor (Ronald Reagan) respectively. So Republicans had actually appointed seven of the nine justices as early as Thomas in 1991. And at that time, Democratic appointee Byron White (John F. Kennedy) was pro-life.
Stevens, O'Connor, and Souter have generally been counted in the liberal wing, particularly on abortion. They all voted to uphold Roe v. Wade. Kennedy is considered more of a swing vote and has voted to uphold Roe v. Wade, although he is more willing to limit it than the more liberal members of the court.
Prior to Scalia's death there were four pro-life justices, four pro-choice justices, and Kennedy. That had been true since O'Connor was replaced by Alito. It was also true before Ruth Bader Ginsburg (William Clinton) replaced White. Presumably it will be true again after Donald Trump appoints a replacement for Scalia.
Kennedy's abortion views are complex. While he might vote pro-life on the right case, he has supported pro-choice positions in the past. There is little reason to think that he is likely to significantly change the system he helped set up in the late eighties and early nineties.
That situation could change if Kennedy or one of the four more liberal justices was replaced by a more conservative justice. The three oldest justices are Ginsburg, Kennedy, and Stephen Breyer. Like Scalia (who had been the second oldest justice), they could be forced off the court by death or bad health. It's not entirely clear who Trump would appoint in that case. There's a list of potential replacements for Scalia, but he won't necessarily be limited by that list in subsequent appointments.