Is it safe to downgrade Lion from 10.7.1 to 10.7?

Question

Questions

Is it OK to downgrade 10.7.1 Client to 10.7 using the original version 1.0.4 of Install Mac OS X Lion.app?

Either type of downgrade to be followed by updates. These questions focus on:

• the period during which a downgraded system runs.

(Installation to a clean volume, without downgrade may be preferable, but that's a separate discussion.)

Key points

Version 1.0.4 of the installer recognises that something more recent than 10.7 (Build 11A511) is running, but:

• does not caution the user against downgrade
• does not create /Previous System
• reuses /System

Background

Install Mac OS X Lion.app 1.0.4 for Mac OS X 10.7 (Build 11A511) appeared in the Mac App Store on 20th July 2011. Following advice from Apple, some users set aside a copy of version 1.0.4.

OS X Lion Update 10.7.1 (Client) and OS X Lion Update 10.7.1 (Server) were posted on 16th August 2011.

Install Mac OS X Lion.app 1.0.8 for Mac OS X (Build 11B26) appeared in the Store on 23rd August. If a copy of 1.0.4 is found, the Store routine may update that copy to 1.0.8.

However:

• between 16th and 23rd August, 1.0.8 was not an option
• some users may be unaware of 1.0.8
• some users are aware of 1.0.8 but prefer to retain and reuse 1.0.4 (to install 10.7 instead of 10.7.1, then update).

Answer 1

Downgrade 10.7.1 (Client) to 10.7

Downgrade is possible, apparently without restriction, but not necessarily OK.

My own case

After using 10.7.1 for a few days, a series of events led me to erase my startup volume then use Recovery OS Time Machine to restore from backup. These events left me without the Recovery HD that is required for FileVault 2.

At that time, version 1.0.8 of the installer was not available.

To regain a Recovery OS:

1. avoiding my usual account, I used a different account to log in to 10.7.1 (intention: data integrity — avoid my usual home directory at all stages during a downgrade)

2. I ran version 1.0.4 of the installer on 10.7.1 to install (downgrade to) 10.7

3. avoiding my usual account, I used a different account to log in to 10.7

4. I observed an issue affecting a peripheral display — at least one thing not OK

5. I didn't investigate that issue, didn't seek other issues (intention: spend as little time as possible in a downgraded environment)

6. I ran Software Update, got 10.7.1 and other updates.

More generally

Data relating to the greatest version of an executable may be incompatible with, or lose integrity with, a lesser version.

Incompatibility or loss of integrity may be acceptable for some types of file (examples: cache files, preference files). For other types (example: application support) integrity may be critical.

For the System version/build over which you wish to reinstall, if Apple provides a matching full installer:

• avoid downgrade — prefer to install a matching system.

Application-specific formats that are designed for export and import

To minimise the risks associated with downgrades, I guess that:

• an archive produced by Address Book in 10.7.1 should be usable without issue in 10.7
• an archive produced by iCal in 10.7.1 should be usable without issue in 10.7

– and so on.

Answer 2

A sure-proof way to ensure nothing wrong happens would to be first do a downgrade to Snow Leopard, if you still have the install/recovery disk (and you don't, borrow or buy one), and then reinstalling Lion using the backed-up install lion.app.