I've been worrying if my marketing will still be effective for my self-published books when I market them after almost a year the novels have been released. My plan is to self-publish more novels this year and advertise the old novels with the new novels. Will my marketing for my previously published books be harder to gain sales because it's not a new release anymore? As a reader will this affect your decision of buying the ebook? The reason for not being able to market them right away is because of my situation in COVID 19 which was during the time I was supposed to market my novels last year. Now, I'm doing better and I'm ready to start advertising. Any tips and advice are well-appreciated.
Plenty of books get a rebranding years, even decades after they've been written. Most recently an widely known, the Song of Ice and Fire books were published decades ago, yet one or two years before the show started ten years ago, they got new covers. They got new covers after every subsequent season
The second thing to note, is that rebranding books you've already written give it an up-to-date look, and look far more professional than having different artists work for every cover, of every book you wrote.
Lastly, as EDL mentioned, short answer is no. With all due respect, it is likely that very few people have read your work, so for the reader, it makes zero difference if the books you've written were five, ten, even twenty years old because to them, they are brand new. The only exception to this, is if your books were meant to be published in specific period of time. (imagine all the books written from 2015-2020 predicting the next decade. They were all wrong due to Covid, so there's no need for anyone to read them beyond academic curiosity)
Some marketing tools aren't available to you once a book is on the market. For example, your book will not appear in a list of new publications, and that is a place where readers look for new reading material. Certain outlets and platforms require to be informed way ahead of publication for them to engage in marketing campaigns for you. And physical bookstores will give shelf and window space only to either new books or bestsellers. And unlike in a rebranding campaign, you don't have reader reviews, media feedback, besteller list status etc. to show.
Marketing your book without these tools will make marketing more expensive and effortful for you, but it is certainly possible if you are willing to make the investment. All other channels (paid advertising, querying professional reviewers, etc.) are still open to you.