We have performed a range of in situ heating experiments of polycrystalline Bi films of 22-25 nm-thickness in a transmission electron microscope (TEM). This shows that it is possible to locally transform a polycrystalline thin film into a -oriented single-crystalline film, whereby the unique feature is that the original thickness of the film is maintained, and the substrate used in our experiments is amorphous. The single-crystalline areas have been created by heating the Bi film to temperatures close to the melting temperature with additional heating by focusing of the electron beam (e-beam), which results in local melting of the film. The film does not collapse by dewetting, and upon subsequent cooling, the film transforms into a single-crystalline  oriented area. The observed phenomenon is attributed to the presence of a thin Bi-oxide layer on top of Bi film. We show that removal of the Bi-oxide layer by heating the film in a H2 gas atmosphere results in changes in the Bi film thickness and dewetting upon in situ heating in the TEM.