'제로미' is not considered to be a change in its form- because we can notice that the writer was intending to spell '제롬이' in the... wrong way. '제로미' is rather a way to show friendliness between friends, especially among younger generations.
This is some information that might help you understand further.
'제로미' might look new to you because it is written as how it sounds like, not following the spelling rule. Korean words are not spelled in the exactly same way it is pronounced. In your examples they are mostly about to the rule of linking. Let me take an example sentence '꽃이 되리라'. Though this is pronounced to be '꼬치 되리라', spelling should remain its original form before linking. In your examples of 지미니 and 제로미, this rule of linking is broken- intended to sound more friendly.
This friendliness-showing texts can appear at any place where linking takes place. For example,
'제롬이랑 밥 먹으러 가자!'
Can be written as
'제로미랑 밥 머그러 가자!!!~ >_<★'
Notice the difference? They do contain the same meaning but some of your friends might want to express their... special joy and remove any remaining textbook-ish feeling by taking the second sentence. However don't be disappointed when your friend would not use a sentence like the second one since this is only a matter of choice between different people- in fact some would prefer a fresh and plain unbroken spellings.
Often used by girls, rarely used in age group above 30s. Girlfriend and her boyfriend might talk to each other like this for...애교!!;;
I hope this would help.
For further studying you might want to find out more about 'Korean spelling rules' and the reasons behind them.