Bard and Machinist? Can I switch back between them?

Hello. I'm still really new to FFXIV and even though I'm level 55 I'm still on the main quest which is around lv. 45 or so. I didn't realize I could just mainly level on my class quest and was doing everything for a while lol. Plus with this Yokai event I've really out-leveled my main quests. I'm not going to work on a second job yet but what I wanted to know is if I decide to try Machinist (which sounds really awesome) I can still switch back to a bard just by equipping a bow right? I know all the classes works by changing weapons but I'm not sure about classes like this which seem like an extension of archer. So I just want to make sure before I try it. I started my archer to play a machinist but I've sort of fallen in love with the bard too so I would hate to switch if I can't come back to it.

Welcome to Eorzea!
You posted this in Disciples of the Hand subforum, it is a question better suited to New Player Help subforum. That being said, on to answering your question.
Yes, you can save gear arrangements for multiple classes/jobs. You can switch between these save sets anytime you are not in combat or a dungeon/raid. The ability to play all jobs on a single character profile is one of the greatest design attributes of FFXIV.
On the character profile screen, at the top near your weapon, you will see Page icon, this is where you access Gear Set screen to select and/or save new gear sets. Select a different gear set, and you will change jobs immediately.

Now, as for Machinist, unless they changed it for Stormblood, you cannot unlock it until you have reach the Heavensward part of the story line. As your MSQ is around 45, you have nearly 100 quests to go before you'll get there. This would also apply to Darknight and Astrologian.