You cannot make something that was considered safe unsafe by changing a rule.
"Considered" (which I suppose means "thought to be" or "deemed to be") is probably the crucial word there.
Changes in rules/regs obviously cannot alter 'how safe' something actually is. However rules/regs can be (and sometimes are) revised when it is realised/discovered that something which previously was 'considered' ('thought to be') 'safe' actually wasn't/isn't 'safe' - or if something which previous was 'safe' ceases to be so because of changes in the world.
There are countless examples - unlimited exposure to X-Rays or other radiation, heavy metals (mercury, lead, etc,), asbestos, opium, tobacco, thalidomide etc. were at one time 'considered' to be 'safe', but rules/regs (and advice/practices) were subsequently changed/introduced when such was found not to be the case. Similarly, earthing to metal water pipes was once considered to be 'safe', until rules/regs had to be changed because of the emergence of plastic pipes.
The other thing, of course, that views on the desirable/expected level of 'safeness' vary with time. Even when hazards/risks were known in the past, but felt to be 'acceptable risks', subsequent changes in 'our' expectations have often caused rules/regs to change to reflect the fact that 'we' no longer regard the risks concerned as 'acceptable'
Kind Regards, John