I have Sage Line 50 client manager (2011) and I can read all versions (including instant) of the same or older version number apart from the very oldest (2007 I think).  I think I'm right in saying that all sage data (of the same version) is basically the same, it is just the different parts of the programme are activated depending on the licence purchased.

I think what this means in reality is that your accountant should be able to restore sage instant data into line 50 of the same version numbr or newer.  However you can't then go backwards and restore the same data back into your instant.

If your accountant intends to post any data then he should be using client manager which would allow you to carry on posting as it produces a special file just of his adjustments which you can then import. Otherwise the data is just for his reference purposes.  (I'm afraid most Accountants I have come accross are not that IT savvy and do not use Line 50 Clienat Manager to its full potential anyway- maybe I'm bieng unkind).

I personally wouldn;t go the the espense of buying Line 50 for a very small business, Sage Instant is usually perfectly adequate.

It may be that he just likes the format of the sage reports etc.

Why don't you ask the accountant what he would recommend and why

Although I have used Sage in various forms extensively for may years I do not consider myself to be an expert, there may be others on here who can advise more fully. There is a specific Sage Line 50 Group over on AWeb where there is much excellent advice. But I hope this helps a little