The word here translated "the holiest of all" is `tou hagiou', which is the Neutral, Singular Genitive for both, `tou' and `hagiou', of the root word `ta hagio-'. According to the definition in Hebr. 9:1, the singular refers to the sanctuary as a whole, which is a bit strange since for two rooms the singular is used and later, for one room, the plural. Thus the colon, ":", at the end of verse 7 in the KJV ought to be a period because the thought presented in verse 7 ends right there. Vers 8 is not a continuation of the thought on the `second' apartment in verse 7. The AKJV does also have a colon; while the RSV does not.

Since in this verse the Neutral, Plural Nominative is used for both `ta' and `hagia', we have hear the a bit strange case where a single room is refered to by a plural noun. But in this case the rendition we find here in verse 12 presents no problem and may be retained as long as "holy place" is the first apartment.

The aparatus in the Greek Bible has at this place the following: "The `' symbol, meaning some included something here and they show, (a) Trn agin P.
What does that mean? The P means either of two sources, the (1) Wolfenbuettel (6the cent.) or (2) the Porfirianus (at Leningrad) (10th cent.) source.
That means it seems save to ignore either of the two sources. We just bring it up to be thorough.

The rendition in this is also based on `hagia', the Neutral, Plural Nominative which would refer to the first apartment. Some translators seem to be inclined, since God is present there, to opt for the idea that it then must be the Most Holy. But be reminded, in Revelation chapter 4:1 to 11:18, God Father is shown to be in the first apartment, the holy place or room, by the type of furniture present - the seven branch candle stick (ch. 4:5) and the golden altar of incence (ch. 8:3). Not until Rev. 11:19 is the door to the Most Holy apartment opened.

There is no change indicated here since this verse uses `ta hagia' the Neutral, Plural Nominative which would be the first apartment, the holy room.

The aparatus for verse 25 also has the interpolation sign. In this case we see there exactly this: "tn agin 69 1912 pc sa". {`twn hagiwn' would be the Neutral Plural Genitive.} The `69' means some extract of a preached presentation in 1912. The `p' means `and parallels', no parallels are seen here, but perhaps in a 1912 edition. The c means `capitulum', a margin text - there is none. The `sa' means the Sahidic, originating in Upper (south) Egypt in the 2nd or 3rd century and received in fragments dating from the 4th century onwards. - Thus it seems we can safely ignore these aparatus information bits.

KJV: Thus we see that in the KJV the only change of meaning would occur in Hebrews 9:8. The way the KJV has it, Christ would skip the real priestly ministry in the holy apartment after His ascension and right away serve in the Most Holy apartment where it not for the "not" in verse 8. However, using the definitions, not even the way into the first apartment was yet made manifest to the Christians (or Jews) of the early, apostolic church because the Temple in Jerusalem was still standing and it took time for them to realize that there is a sanctuary in heaven.
AKJV: Has "Holiest of all" in verse 8. Verses 12 & 25 have "holy place" and v. 24, "holy places".
RSV: In this version we have in verse 8 "the sanctuary", in verse 12 "the Holy Place", v. 24 "a sanctuary" and in v. 25 "the Holy Place".
ESV: Has "holy places" in verses 8,12,24,25. Thus it is suspect, since it sees in every one of these verses as if it is speaking of the whole sanctuary, thus not differentiating the two apartment services of Christ, first as Priest in the first apartment, followed as High Priest in the Most Holy, thus fulfilling what the Levitical priesthood foreshadowed.
NIV: This is the same way also in the NIV for Hebr. 9:8 and 12 and 25. Verse 24 is ok in the NIV.
The Living Bible: Same as the NIV.
Moffatt: Has "holiest of all" in verse 8, the others are ok.
New World Translation: Has "holy place" in all these four verses, therefore it is ok. The other versions capitalize "Holy Place" leaving a degree of ambiguity that they may mean the Most Holy, may we assume the first apartment deserves capital letters just the same(?).

Thus we see that all versions are suspect, yet amazingly enough, the New World Translation version is ok on this chapter (perhaps not realizing the impact) - the other versions leave out Christ's ministry in the first apartment of which the daily services in the Levitical priesthood were a shadow - thus the real service is not taught and Christ's gospel ministry diminished, if not done away with for the part taking place in the first apartment, by extension then also affecting what Christ would do in His High Priestly ministry in the Most Holy in the `Day of Atonement' real service which started in 1844, not after His ascension.
{The SOP does not quote Hebr. 9:8.}