It is possible to "bump up" bullet diameters, as well as reforming specific portions of a bullet to different shapes or profiles. The requirements are a swaging die of the desired shape, diameter, etc, and a press capable of applying sufficient pressure (measured in multiples of tons) to force the projectile into the new form.
This is a gross simplification of the process, of course. Other factors that would come into play include the hardness of the projectile (both core and jacket, as applicable), with the harder materials having a tendency to fracture rather than flow under pressure, and limitations on the capabilities of die forms to both receive and form the piece while also allowing the resulting piece to be extracted without damage (lubricants may be useful, but may also adversely affect the forming process).
Swaged bullets have been in use for well over a century. One application was during the US Civil War with "picket rifles" (heavy rifles set up on posts or pickets and used for long range engagement of individual targets). These rifles utilized bullets made by casting an undersized slug of lead, then driving it into a swaging die to final form and diameter that precisely matched the bore and grooves of the rifle. Such rifles were very precisely made, no mass production was possible for the degree of accuracy required, and no commonality of parts was usually found, each rifle was delivered with an appropriate casting mold and swaging tool.
Modern swaged bullets include both jacketed and unjacketed types. Undersized pieces of pure lead (or very mile alloys) are formed under high pressures, then ejected from the forming dies (typically adjusted to carefully control final results to close tolerances of weight in addition to form and contours). When jackets are used they are typically slightly undersized cup-shaped pieces into which the lead core material is inserted, then the swaging process is done (frequently in multiple stages to achieve a final form). At least one maker (Speer) offers jacketed bullets with the interior of the jacket cups tinned with solder, then the lead core material inserted at or near liquid state, then swaged together resulting in a strong bond to hold the core and jacket together as one piece (the goal being to allow controlled expansion while keeping the bullet in one piece to enhance penetration).
Bullet swaging is a highly advanced aspect of the shooter and reloader sports. There have been any number of top-level competitors (particularly bench rest shooters) that have invested heavily in the equipment to create their own bullets to their own specifications. Demand for this type of equipment has steadily declined over the past 50 years or so as more sources of factory-made bullets of extremely high quality and consistency have become available to the general public.
The hobbyist will always be limited by budget and availability of a facility capable of operating the necessary machinery to produce consistent results. While a large manufacturer may have industrial space and expensive equipment, the home producer is probably limited in equipment and facilities.