Is there any significant difference other then strings between "regular acoustic guitar" and an "acoustic lap steel guitar"?
Is replacing the strings with steel strings enough?
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A purpose-built lap steel guitar has its strings typically much further from the fretboard than a conventional guitar. So much so, that it's extremely difficult to press the string to the fretboard: the frets are just for reference. Contact with the slide is how you get pitch.
Having said that, you can certainly get close to a lap steel sound using a conventional guitar. You'll need a precise touch, to get solid enough contact with the strings, without pressing onto the frets.
Lap steel players use open chord tunings such as D-G-D-G-B-D. If you intend to leave a conventional guitar in such a tuning for long periods, then it's wise to buy a set of strings designed for that tuning.
Conventional non-classical acoustic guitars already have steel strings. Do not under any circumstances fit steel strings to a guitar designed for nylon strings -- if the tension doesn't snap your guitar's neck, it will certainly bend it beyond repair. Guitars designed for steel strings have a metal truss rod providing the strength it needs.
Different instrument groupings going on here:
You can, by getting a nut extender and tuning to an open tuning, turn a spanish style electric or acoustic guitar into a lap-style guitar. There are a number of differences in purpose-built lap-style instruments that you will want to consider. Primarily, the string spacing and height of spanish guitars are there to fit in the hand, and that is not what you want when playing lap steel. As a stop-gap until you get what you want, it works.
If you're wanting to play lap steel, that's about a good stopping point. If you're wanting to play resonator style, like Jerry Douglas and Brother Oswald and Mike Auldridge, throwing on a nut extender and tuning GBDGBD is only going to get you so far. The resonator setup was invented by the Dopyera Brothers (Do-Bros, thus dobro) in order to amplify, and amplifying an instrument changes the charactaristics of the tone. You will not get the bluegrass player sound without having a resonator instrument. You can get a sound that works for Hawaiian music and might have other uses and be fun, but it isn't the sound you probably want.