To the best of my knowledge there are no published reports on this aspect. Why are you stressing on biosynthesized silver nanoparticles? Do you have information on this aspect with regard to silver nanoparticles synthesized through physical and chemical methods? or do you want to know impact of freezing on biosynthesis of silver nanopareticles? Temperature does influence synthesis, shape and stability of silver nanoparticles. For example at higher temperature under oxygenic conditions, rate of oxidation of silver nanoparticles will increase and accordingly, you can expect large proportion of Ag2O nanopaticle and even agglomeration rate may increase. 

As compared to chemically synthesized nanoparticles, the stabilizer is unknown in case of biosynthesized nanoparticles. Also the stabilizer may be different for different nanoparticles in the solution.

If the stabilizer is a protein then proteins can get denatured at low temps (-200C or -800C) and come off the nanoparticles making them prone to aggregation.

But even if the stabilizer comes off the nanoparticle, the nanoparticles would not aggregate at low temperatures -200C or -800C because the sample is frozen and nanoparticles cannot  come close for aggregation to occur. When the frozen sample is brought back to room temperature, then the nanoparticles can aggregate leading to size and/or shape changes.

Storage at 40C should not make much of a difference.

At low temperature, there is a chance of shape transformations in some systems. This is a thermodynamic phenomena. If kinetically the transformations are favorable, then they can take place. Need to check up if there are any reports about the same in the literature for silver nanoparticles.

Excellent answer V. N. Ravi Kishore V.