Modification and replacement approval
have no professional or expert qualifications of any kind.
Does an A&P or IA have the authority to replace a failing non-TSO Altimeter with a used or new functioning non-TSO altimeter with only a log book entry and without the necessity of FSDO review and/or approval?
First, let's separate out the TSO issue.
FSDO review and approval is only required for major repairs and major alterations not supported by other approved data.
Major repairs and major alterations are defined in part 43 appendix A.
Approved data includes STC's, Type certificate data sheets and aircraft specifications, AD's, FAA approved manufacturer's maintenance manuals, work product of a DER, and field approvals (this is where the FSDO comes in). In addition, for unpressurized aircraft under 12500 lbs for which there is no FAA approved maintenance manual, AC43-13.1C and .2B constitute approved data. (I may have missed one, and I may have gotten the 1C-2B stuff mixed up a little, but you get the idea....)
FAA Approved 337's dated prior to October 1, 1955 constitute approved data and may be used just like an STC to perform the same repair or modification. So, thumb through those old logs, guys! There's gold in there...
All major repairs and major alterations must be recorded on a form 337 and supported by approved data.
By (their) definition, anything that is not a major repair or major alteration is a minor repair or alteration and can be returned to service by an A&P with a log book entry.
Replacement of an approved altimeter with another approved altimeter is a minor repair, hence an A&P and log book entry is sufficient.
Now, we're back to "what constitutes an approved altimeter"? I still feel that the field is not unlimited. I would be comfortable with a used or overhauled altimeter of the same make and model as the original, or a similar make and model used in another approved application, i.e. from a Cessna, Piper, Beech, etc. I think the non-TSO'd UMA unit previously mentioned would probably be acceptable as would any TSO'd unit. I am still not comfortable with the "Chin Wah" unit, the Timex watch, or the unit from Sharper Image.
There was an interesting comment by someone else earlier that a TSO'd part still has to be approved for a particular airframe. If I may cite an example to clarify this: Most aircraft tires are manufactured to a TSO. The Aircraft specification or TCDS calls out a specific size and ply rating. Hence, a TSO'd tire of 600-6 2 ply is acceptable on an Erccoupe but another tire manufactured to the same TSO but of a different size is not approved, aside from the fact that it wouldn't fit...
An aside: Chin Wah Tire Works LTD manufactures a 600-6 2 ply wheel barrow tire marketed through Harbor Freight. Is that approved for the Ercoupe? Not in my book...