p = Plot[Sin[x], {x, 0, 1}]

Show[p, (* Option?? *)]

p = Plot[Sin[x], {x, 0, 1}]
col = Cases[p, _Hue, Infinity][[1]];
Show[p /. col -> Red]

myplot = Plot[{Cos[x], Sin[x]}, {x, 0, 2 Pi},
  PlotStyle -> {{Red, Dashing[None]}, {Green, Dashing[None]}}]

newstyles = Directive @@@ {
    {Green, Thickness[.02], Dashing[Tiny]},
    {Thickness[Large], Red}
  };

i = 1;
MapAt[# /. {__, ln__Line} :> {newstyles[[i++]], ln} &, myplot, {1, 1}]

restylePlot[plot_Graphics, styles_List, op : OptionsPattern[Graphics]] :=
 Module[{x = styles}, Show[
   MapAt[# /. {__, ln__Line} :> {Directive @ Last[x = RotateLeft@x], ln} &, plot, 1],
   op
 ]]

myplot2 = Plot[Evaluate[Table[BesselJ[n, x], {n, 4}]], {x, 0, 10}, Filling -> Axis]

restylePlot[myplot2,
 {
  {Green, Thickness[.02], Dashing[Tiny]},
  {Thickness[Large], Red},
  Blue
 },
 Axes -> False,
 Frame -> True,
 FrameStyle -> Directive[20, FontColor -> Orange]
]

restylePlot2[p_, op : OptionsPattern[Plot]] :=
    ListPlot[Cases[Normal@p, Line[x__] :> x, ∞], op, Options[p]]

restylePlot2[myplot2,
 PlotStyle -> {{Green, Thick, Dashed}, {Thickness[Large], Red}, Blue},
  Filling -> Axis, FrameStyle -> Directive[20, FontColor -> Orange],
 PlotLegends -> LineLegend[{"1", "2", "3", "4"}]]