AttributedStringIssue.playground

import UIKit

struct GuiUtils {
    
    static func setAttributedStringToSize(attributedString: NSAttributedString, size: CGFloat) -> NSMutableAttributedString {
        let mus = NSMutableAttributedString(attributedString: attributedString)
        mus.enumerateAttribute(.font, in: NSRange(location: 0, length: mus.string.count)) { (value, range, stop) in
            if let oldFont = value as? UIFont {
                let newFont = oldFont.withSize(size)
                mus.addAttribute(.font, value: newFont, range: range)
            }
        }
        return mus
    }
}

let initialString = NSAttributedString(string: "AA BB CC DD EE", attributes: [.font: UIFont.systemFont(ofSize: 8.0)])

let changedInitialString = initialString.mutableCopy() as! NSMutableAttributedString
changedInitialString.replaceCharacters(in: NSRange(location: 0, length: 6),
                                       with: NSAttributedString(string: "replaced"))

let good = GuiUtils.setAttributedStringToSize(attributedString: initialString, size: 33.0)
let bad = GuiUtils.setAttributedStringToSize(attributedString: changedInitialString, size: 33.0)