maybe possible with the correct profile tyres to maintain the same diameter to 10%, also 18's will be quite a wide rim like 8 or 9 inch so they may be too wide, unless you mismatch width for front and rears but even 7 inch is quite big for fronts on a celica id say.
you would be buying liqourice strap tyres that cost more. and they handle differently being so thin. and you cannot increase your track more than 26mm so you would be looking for a very exact set of wheels especially in 18's. probably a negative offset if they are quite wide - ie deep dish.
track and diameter are for qld, check for other states if applicable.
and most likely you will have to roll the front guards maybe rears to get clearance.