(Answer is for US jurisdiction)
You're dealing more with trademark law than copyright law. A logo is a trademark, as it is the identifying service mark of a company.
A trademark must be a unique identifying mark, specifically associated
with the goods or services that a company offers in commercial trade.
One type of trademark includes the company logo. To qualify as a
trademark, a logo must be a unique mark used to identify and
distinguish the company's goods or services offered in the
marketplace. Difference Between a Logo & Trademark (LegalZoom)
So: could your conceptual/redesigned logos be a problem for the real companies and their tradmerks? Maybe so.
How? It's important to determine if there a possible "Likelihood of confusion" between your designs and the trademarked logos.
A likelihood of confusion exists when consumers viewing the allegedly
infringing mark would probably assume that the product or service it
represents is associated with the source of a different product or
service identified with a similar mark. Trademark infringement (Wex Legal Dictionary)
If that company determined themselves that there was a "Likelihood of confusion", that could be one of the central arguments they would use to began a trademark infringement action against you.
I understand that the logo will be in a portfolio of work not intended to be commercial and not intended to cause confusion, as stated in a disclaimer.
But, any of what is called a "Likelihood of confusion" is a possible strike against you, and would be determined in court. Your possible defense, as a designer and graphic artist, could be "Fair Use":
...an alleged infringer may assert the defenses of fair use.... Fair Use (Wex Legal Dictionary) allows commentary or criticism that incidentally
involves the use of a trademark so long as such use is for a purpose
other than that normally made of a trademark.
You are using the trademark for inspiration and commentary outside of the traditional uses of marketing and company identity.
A disclaimer would also help illustrate that your portfolio is made up of conceptual samples of graphics work and not commissioned work and not real trademarks; it can show that you understand that your graphic work is not intended to be confused with the real trademarks. But very importantly, a disclaimer will not absolve you of legal liability.
But, the fact remains that you have done a conceptual/redesigned logo for that company, and if that design is close enough - colors, design, name (or fictionalized name) of company - for casual observers to possibly be confused, then it is possible to draw legal action against yourself, even if that trademark holder realizes that the designs are conceptual and your disclaimer explains your position.