How possible is it for antibiotic resistant bacteria stored for a long time to lose such resistance?
It is possible due to the fact that some antimicrobial resistance genes are induced by the presence of antibiotics themselves and are not promoted in the absence of a specific antibiotic, so the production of certain antimicrobial structures (enzymes, pumps, etc.) will be reduced once the bacterial medium is antibiotic free for enough time. This is sometimes refered to as "antibiotic pressure" referencing to some extent the genomic concept of selection pressure.
Take for example the phenomena reported in this paper's summary; they studied a patient hospitalized because of an infection by 2 K. pneumoniae strains. They knew beforehand that one of the strains was suceptible to carbapenems while the other one had a certain degree of resistance to this antibiotic. As the infection unfolded and a treatment with meropenem was administered, they took several samples and studied them via whole genome sequencing (WGS). They found out that the strain previously characterized as suceptible to carbapenems had acquired a plasmid (possibly by vertical transmission from the other strain) and hence now exhibited resistance to carbapenems, however when this strain was left in observation (and without being exposed to meropenem) it lost its previously acquired resistance.
In conclusion the sole presence of antibiotics induces resistance by different mechanisms in bacteria, but in some cases the lack of antibiotics in the medium can also revert resistance if this condition lasts for enough time. It is important to mention though that some resistance profiles will never be reverted no matter for how long the bacteria remains away from an antibiotic due to their nature. For example a gram-negative bacteria will always have instrinsic resistance to regular penicillin no matter what due to its cell structure itself (lack of a thick peptidoglican cell wall and presence of an outer cell membrane).