What is a term describing offensive word replacement with made-up slang? [duplicate]

Solution 1:

The general term for these is euphemism.

The use of a word or phrase to replace another with one that is considered less offensive or less vulgar than the word or phrase it replaces.

Edit: just found another term: minced oath. I've never actually heard this term used, mind you, but still, it's a closer match than plain "euphemism".

Solution 2:

I would call this a minced oath:

an expression based on a profanity or a taboo term that has been altered to reduce the objectionable characteristics.

Lots of fun stuff like "strewth", "cheese and rice"...

Solution 3:

The basic process is called euphemizing (replacing a "harsh" word with a "softer" word or phrase).

Solution 4:

Q: Is there a term for replacing one slang word with another made-up slang word?

In addition to euphemism, the more general term is bowdlerisation (to bowdlerise, bowdlerised):

From Thomas Bowdler who in 1818 published a censored version of Shakespeare, expurgating "those words and expressions [...] which cannot with propriety be read aloud in a family."

1. To remove those parts of a text considered offensive, vulgar or adult in nature.
   the bowdlerised version of the text, while free of vulgarity, was also free of flavour
2. (by extension) To remove those parts of a text considered to be damaging to an authority.

You can see lots of examples at TV Tropes.

Q: At what point do the made up words turn into recognized/official words? For example I'm pretty sure I can use most of the above replacements and people will know what I mean, fricking and fracking being most used.

English has no official arbiter of words, so it depends what you mean by recognised and official. It just depends on usage. Once a word is used widely enough then almost by definition, most people will understand it.

Q: Even if a replacement becomes commonly used, at what point would it end up in say the OED?

It doesn't really matter if the words are slang or offensive, it again depends on usage. The OED has a FAQ on how a word qualifies for inclusion. Briefly, if a word has been used by enough people, and/or for long enough, then it's in.

Of your examples, fricking, fracking and freaking are already in Oxford Dictionaries Online as vulgar slang used as euphemisms for fucking. Smurfing and frelling aren't there, but these are less well known.